Nitride semiconductor device and fabrication method therefor

ABSTRACT

A nitride semiconductor device includes an electron transit layer (103) that is formed of a nitride semiconductor, an electron supply layer (104) that is formed on the electron transit layer (103), that is formed of a nitride semiconductor whose composition is different from the electron transit layer (103) and that has a recess (109) which reaches the electron transit layer (103) from a surface, a thermal oxide film (111) that is formed on the surface of the electron transit layer (103) exposed within the recess (109), a gate insulating film (110) that is embedded within the recess (109) so as to be in contact with the thermal oxide film (111), a gate electrode (108) that is formed on the gate insulating film (110) and that is opposite to the electron transit layer (103) across the thermal oxide film (111) and the gate insulating film (110), and a source electrode (106) and a drain electrode (107) that are provided on the electron supply layer (104) at an interval such that the gate electrode (108) intervenes therebetween.

FIELD OF THE ART

The present invention relates to a semiconductor device formed of agroup-III nitride semiconductor (hereinafter also simply referred to asa “nitride semiconductor”) and a method of manufacturing the same.

BACKGROUND ART

A group-III nitride semiconductor is a group III-V semiconductor inwhich nitrogen is used as a group-V element. Representative examples ofthe nitride semiconductor include aluminum nitride (AlN), galliumnitride (GaN) and indium nitride (InN). In general, the nitridesemiconductor can be represented as Al_(X)In_(Y)Ga_(1-X-Y)N (0≤X≤1,0≤Y≤1, 0≤X+Y≤1).

A HEMT (high-electron-mobility transistor) using such a nitridesemiconductor is proposed. A HEMT includes, for example, an electrontransit layer that is formed of GaN and an electron supply layer that isepitaxially grown on the electron transit layer and that is formed ofAlGaN. A pair of a source electrode and a drain electrode are formed soas to be in contact with the electron supply layer, and a gate electrodeis arranged therebetween. The gate electrode is joined to the electronsupply layer with a Schottky junction or is arranged so as to beopposite to the electron supply layer across an insulating film. Due topolarization caused by the lattice mismatch between GaN and AlGaN,within the electron transit layer, in a position a few angstroms inwardfrom an interface between the electron transit layer and the electronsupply layer, a two-dimensional electron gas is formed. Thetwo-dimensional electron gas is used as a channel, and thus a connectionis made between the source and the drain. When a control voltage isapplied to the gate electrode to interrupt the two-dimensional electrongas, an interruption occurs between the source and the drain. Since in astate where the control voltage is not applied to the gate electrode,electrical continuity is established between the source and the drain,the device functions as a normally on-type device.

Since the device using the nitride semiconductor has features such as ahigh-voltage resistance, a high-temperature operation, a high-currentdensity, high-speed switching and a low on-resistance, the applicationto power devices is being examined.

However, since the device needs to be a normally off-type device whichinterrupts a current at the time of zero bias for an application for apower device, the HEMT described above cannot be applied to the powerdevice.

A structure for realizing a normally off-type nitride semiconductor HEMTis proposed in, for example, Patent Document 1.

On the other hand, Patent Document 2 discloses a GaN-based semiconductordevice that is produced as follows: a plurality of GaN-based HEMTs(high-electron-mobility transistors) are formed on a silicon substrateand the electrodes of the GaN-based HEMTs are coupled to each other bymultilayer wiring. On the silicon substrate, a buffer layer and asemiconductor operation layer are formed. The semiconductor operationlayer is separated into a plurality of semiconductor operation layerregions by an insulating region formed by ion implantation.

PRIOR ART DOCUMENT Patent Document

Patent Document 1: Japanese Patent Application Publication No.2011-171440

Patent Document 2: Japanese Patent Application Publication No.2008-235740

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda nitride semiconductor device including: an electron transit layer thatis formed of a nitride semiconductor; an electron supply layer that isformed on the electron transit layer, that is formed of a nitridesemiconductor whose composition is different from the electron transitlayer, and that has a recess which reaches the electron transit layerfrom a surface; a thermal oxide film that is formed on a surface of theelectron transit layer exposed within the recess; a gate insulating filmthat is embedded within the recess so as to be in contact with thethermal oxide film; a gate electrode that is formed on the gateinsulating film, and that is opposite to the electron transit layeracross the thermal oxide film and the gate insulating film; and a sourceelectrode and a drain electrode that are provided on the electron supplylayer at an interval such that the gate electrode intervenestherebetween. The nitride semiconductor device has a normally off-typeHEMT structure, and excellent device properties.

According to a second aspect of the present invention, there is provideda nitride semiconductor device including: a substrate; an electrontransit layer that is formed on the substrate, and that is formed of anitride semiconductor; an electron supply layer that is formed on theelectron transit layer, and that is formed of a nitride semiconductorwhose composition is different from the electron transit layer; an AlGaNbuffer layer that intervenes between the substrate and the electrontransit layer, and that includes a high aluminum composition regionwhose aluminum composition is relatively high and a low aluminumcomposition region whose aluminum composition is lower than the highaluminum composition region and which is arranged in a region close tothe electron transit layer as compared with the high aluminumcomposition region; and an element separation layer that is formed witha region whose resistance is increased by causing a crystal defectthrough ion implantation and that passes through the electron supplylayer and the electron transit layer to reach the AlGaN buffer layer.With the nitride semiconductor device, it is possible to effectivelyreduce the leak current.

The objects, features and effects of the present invention describedabove or further other objects, features and effects will be furtherclarified by the following description of preferred embodiments withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view for illustrating theconfiguration of a nitride semiconductor device according to a preferredembodiment of the present invention.

FIG. 2 is a plan view of the nitride semiconductor device.

FIG. 3 is an enlarged cross-sectional view showing an example of astructure in the vicinity of a recess in an actual element of thenitride semiconductor device.

FIG. 4A is a schematic cross-sectional view for illustrating an exampleof a step of manufacturing the nitride semiconductor device.

FIG. 4B is a schematic cross-sectional view showing the step subsequentto FIG. 4A.

FIG. 4C is a schematic cross-sectional view showing the step subsequentto FIG. 4B.

FIG. 4D is a schematic cross-sectional view showing the step subsequentto FIG. 4C.

FIG. 4E is a schematic cross-sectional view showing the step subsequentto FIG. 4D.

FIG. 4F is a schematic cross-sectional view showing the step subsequentto FIG. 4E.

FIGS. 5A to 5C are enlarged cross-sectional views showing structureshalfway through the manufacturing step in the vicinity of the recess inthe actual element of the nitride semiconductor device.

FIG. 6A is a characteristic diagram obtained by examining a relationship(Ids-Vds characteristics) between a drain current Ids and a drainvoltage Vds for various gate voltages Vgs.

FIG. 6B shows transfer characteristics.

FIG. 6C shows voltage resistance characteristics (3-terminal off-statecharacteristics) in an off-state.

FIG. 7 shows variations in threshold value Vth for a plurality ofsamples (example and a comparative example) of the nitride semiconductordevice.

FIG. 8 is a schematic cross-sectional view (cross-sectional view takenalong line VIII-VIII in FIG. 13) for illustrating the configuration of anitride semiconductor device according to a preferred embodiment of thepresent invention.

FIG. 9 is a schematic plan view for illustrating the overallconfiguration of a chip of the nitride semiconductor device according tothe preferred embodiment, and shows the configuration by seeing throughan interlayer insulating film.

FIG. 10 is a schematic plan view for illustrating the arrangement of anelement separation layer and element regions.

FIG. 11 is a schematic plan view for illustrating the arrangement of asource wiring film and a drain wiring film.

FIG. 12 is an enlarged plan view of a part of FIG. 9, and shows theconfiguration by seeing through the interlayer insulating film as inFIG. 9.

FIG. 13 is a partially enlarged plan view for illustrating aconfiguration within the element region.

FIG. 14A is a band diagram showing energy levels at individual parts inthe direction of the thickness of the nitride semiconductor device.

FIG. 14B is a similar band diagram on the structure of the comparativeexample where an AlGaN buffer layer is formed with only an AlGaN layerof high aluminum composition.

FIG. 14C is a similar band diagram when in the comparative example, thethickness of the AlGaN buffer layer formed with a single layer of theAlGaN layer is decreased and the thickness of an electron transit layeris increased.

FIGS. 15A, 15B and 15C are optical micrographs of the surface of a GaNlayer epitaxially grown to various thicknesses.

FIG. 16A is a cross-sectional view showing a step of manufacturing thenitride semiconductor device.

FIG. 16B is a cross-sectional view showing a step subsequent to FIG.16A.

FIG. 16C is a cross-sectional view showing a step subsequent to FIG.16B.

FIG. 16D is a cross-sectional view showing a step subsequent to FIG.16C.

FIG. 16E is a cross-sectional view showing a step subsequent to FIG.16D.

FIG. 16F is a cross-sectional view showing a step subsequent to FIG.16E.

FIG. 16G is a cross-sectional view showing a step subsequent to FIG.16F.

FIG. 16H is a cross-sectional view showing a step subsequent to FIG.16G.

FIG. 16I is a cross-sectional view showing a step subsequent to FIG.16H.

FIG. 17 is a diagram for illustrating the details of a multistage ionimplantation for forming an element separation layer.

FIG. 18 is a diagram showing leak characteristics in an off-state.

MODES FOR CARRYING OUT THE INVENTION

The features of a semiconductor device according to a first preferredembodiment of the present invention are as follows.

A1. This preferred embodiment provides a nitride semiconductor deviceincluding: an electron transit layer that is formed of a nitridesemiconductor; an electron supply layer that is formed on the electrontransit layer, that is formed of a nitride semiconductor whosecomposition is different from the electron transit layer, and that has arecess which reaches the electron transit layer from a surface; athermal oxide film that is formed on a surface of the electron transitlayer exposed within the recess; a gate insulating film that is embeddedwithin the recess so as to be in contact with the thermal oxide film; agate electrode that is formed on the gate insulating film, and that isopposite to the electron transit layer across the thermal oxide film,and the gate insulating film and a source electrode and a drainelectrode that are provided on the electron supply layer at an intervalsuch that the gate electrode intervenes therebetween.

In this configuration, on the electron transit layer, the electronsupply layer whose composition is different is formed to form aheterojunction. Hence, within the electron transit layer in the vicinityof the interface between the electron transit layer and the electronsupply layer, the two-dimensional electron gas is formed, and the HEMTutilizing the two-dimensional electron gas as a channel is formed. Inthe electron supply layer, a recess (convex portion) reaching, from itssurface, the electron transit layer is formed. Hence, at the bottomportion of the recess, the heterojunction is interrupted, and thetwo-dimensional electron gas is interrupted accordingly. On the otherhand, the gate insulating film is embedded within the recess, and acrossthe gate insulating film, the gate electrode is opposite to the electrontransit layer. Then, the source electrode and the drain electrode arearranged at an interval with the gate electrode interveningtherebetween. Since the two-dimensional electron gas is interrupted inthe vicinity of the bottom portion of the recess, at the time of zerobias when no bias is applied to the gate electrode, an interruption ismade between the source and the drain. Hence, a normally off-type deviceis achieved. On the other hand, when an appropriate on-voltage(specifically, a positive bias) is applied to the gate electrode, sincea channel is formed by electrons attracted to the vicinity of therecess, the two-dimensional electron gases on both sides of the gateelectrode are connected, with the result that electrical continuity isestablished between the source and the drain.

In this preferred embodiment, on the surface of the electron transitlayer exposed within the recess, the thermal oxide film is formed, andthe gate insulating film is in contact with the thermal oxide film. Thethermal oxide film is formed by thermally oxidizing the bottom portionof the recess. In the process of the thermal oxidization, the damage onthe surface of the electron transit layer exposed at the bottom portionof the recess is removed. More specifically, when the recess reachingthe electron transit layer is formed by etching (for example, dryetching), the surface of the electron transit layer is damaged (etchingdamage). This damage causes a decrease in electron mobility anddeteriorates the device properties. However, in the configuration ofthis preferred embodiment, in the process of forming the thermal oxidefilm on the surface of the electron transit layer, the damage is cured.More specifically, the thermal oxide film is formed from the damagedsurface toward the inside of the electron transit layer, andconsequently, the interface between the thermal oxide film and theelectron transit layer is located in a region which is not damaged.Then, since a channel is formed on the interface that is not damaged,the electron mobility in the channel becomes an original or inherentvalue of the nitride semiconductor forming the electron transit layer.

As described above, the recess formed in the electron supply layer ismade to reach the electron transit layer, and thus it is possible toreliably achieve a normally off-type, and it is possible to provide thenitride semiconductor device of the HEMT structure where the electronmobility is high in the channel.

Patent Document 1 discloses a structure in which the thickness of theelectron supply layer is reduced immediately below the gate electrode.However, it is difficult to accurately control the thickness of theelectron supply layer by etching, and it is impossible to completelyremove the electron supply layer immediately below the gate electrode.Hence, since the channel produced by polarization caused by the latticemismatch between the electron supply layer and the electron transitlayer cannot be completely removed, in actuality, a normally off-typedevice is not achieved. In order to achieve a normally off-type device,etching from the electron supply layer is performed beyond the interfacebetween the electron supply layer and the electron transit layer, andthus the interface between the electron supply layer and the electrontransit layer is damaged, with the result that the electron mobility issignificantly reduced and the device properties are significantlydeteriorated.

In this preferred embodiment, the combination of the electron supplylayer/electron transit layer may be any one of AlGaN layer/GaN layer,AlGaN layer/AlGaN layer (where Al composition is different), AlInNlayer/AlGaN layer, AlInN layer/GaN layer, AlN layer/GaN layer and AlNlayer/AlGaN layer. More generally, the electron supply layer may includeAl and N in its composition. The electron transit layer may include Gaand N in its composition, and the composition (in particular, Alcomposition) is different from the electron supply layer. The electronsupply layer and the electron transit layer are different in composition(in particular, Al composition), and thus a lattice mismatch occurstherebetween, with the result that a two-dimensional electron gas causedby polarization is produced within the electron transit layer near theinterface.

A2. The thermal oxide film may contain Ga and O. When the electronsupply layer has a composition including Ga, the thermal oxide film hasa composition including Ga and O.

A3. In the thermal oxide film, the oxygen concentration in the film mayhave a gradient with respect to the direction of thickness of the film.When the thermal oxide film is formed in an atmosphere of oxygen bythermal processing, the oxygen concentration in the thermal oxide filmhas a gradient with respect to the direction of thickness of the film.

A4. The oxygen concentration in the thermal oxide film may be maximum inan interface with the gate insulating film, and may be decreased towardthe electron transit layer.

A5. The maximum oxygen concentration in the thermal oxide film may be10²⁰ cm⁻³ or less.

A6. The film thickness of the thermal oxide film may be less than thefilm thickness of the gate insulating film. In this way, the gateinsulating film can have a necessary film thickness.

A7. The film thickness of the gate insulating film may be approximatelytwice as much as the film thickness of the thermal oxide film.

A8. The film thickness of the thermal oxide film may be 1 to 100 nm.

A9. The thermal oxide film may include a first part that is formed onthe surface of the electron transit layer exposed within the recess anda second part formed on a surface of the electron supply layer exposedwithin the recess, and the first part may be different from the secondpart in film thickness. Within the recess, the thermal oxide film may becontinuously formed from the surface of the electron transit layer tothe surface of the electron supply layer. In this case, since theelectron transit layer and the electron supply layer are different incomposition, the growth rate when the thermal oxide film is grown on thesurface is different. Hence, the thermal oxide film has different filmthicknesses in the first part formed on the surface of the electrontransit layer and the second part formed on the surface of the electronsupply layer.

A10. According to this preferred embodiment, there is provided a nitridesemiconductor device including: an electron transit layer that is formedof a nitride semiconductor; an electron supply layer that is formed onthe electron transit layer, that is formed of a nitride semiconductorwhose composition is different from the electron transit layer, and thathas a recess which reaches the electron transit layer from a surface; agate insulating film that is embedded within the recess; a gateelectrode that is formed on the gate insulating film, and that isopposite to the electron transit layer across the gate insulating filmand a source electrode; and a drain electrode that are provided on theelectron supply layer at an interval such that the gate electrodeintervenes therebetween, where the concentration of each of B, Cl and Siin a surface of the gate insulating film on an opposite side to the gateelectrode is 10²⁰ cm⁻³ or less.

In this configuration, on the electron transit layer, the electronsupply layer whose composition is different is formed to form aheterojunction. Hence, within the electron transit layer in the vicinityof the interface between the electron transit layer and the electronsupply layer, the two-dimensional electron gas is formed, and the HEMTutilizing the two-dimensional electron gas as a channel is formed. Inthe electron supply layer, a recess reaching, from its surface, theelectron transit layer is formed. Hence, at the bottom portion of therecess, the heterojunction is interrupted, and the two-dimensionalelectron gas is interrupted accordingly. On the other hand, the gateinsulating film is embedded within the recess, and the gate electrode isopposite the electron transit layer across the gate insulating film. Thesource electrode and the drain electrode are arranged at an interval soas to sandwich the gate electrode. Since the two-dimensional electrongas is interrupted in the vicinity of the bottom portion of the recess,at the time of zero bias when no bias is applied to the gate electrode,an interruption is made between the source and the drain. Hence, anormally off-type device is achieved. On the other hand, when anappropriate on-voltage (specifically, a positive bias) is applied to thegate electrode, since a channel is formed by electrons attracted to thevicinity of the recess, the two-dimensional electron gases on both sidesof the gate electrode are connected, with the result that electricalcontinuity is established between the source and the drain.

In this preferred embodiment, each of the concentrations of B, Cl and Siin the surface of the gate insulating film on the opposite side to thegate electrode is 10²⁰ cm⁻³ or less. When in the electron supply layer,the recess is formed by dry etching (for example, plasma etching), asthe etching gas, a gas (for example, BCl₃ or SiCl₄) including at leastone of B, Cl and Si is used. In this case, when after the etching,thermal oxidization processing (for example, at 900° C. or more in anatmosphere of oxygen) is performed on the electron transit layer exposedwithin the recess, B, Cl and Si included in the etching gas react withoxygen and are scattered, with the result that the concentration of B,Cl and Si left on the surface is 10²⁰ cm⁻³ or less. In other words,after the etching, thermal oxidization is performed on the surface ofthe electron transit layer within the recess, and consequently, theconcentration of B, Cl and Si in the surface of the gate insulating filmon the opposite side to the gate electrode is 10²⁰ cm⁻³ or less. This issaid to be a trace that selective thermal oxidization processing isperformed on the exposed surface of the electron transit layer.

In the process of the thermal oxidization, the damage on the surface ofthe electron transit layer exposed at the bottom portion of the recessis cured. More specifically, when the recess reaching the electrontransit layer is formed by etching (for example, dry etching), thesurface of the electron transit layer is damaged (etching damage). Thisdamage causes a decrease in electron mobility and deteriorates thedevice properties. However, in the configuration of this preferredembodiment, in the process of thermally oxidizing the surface of theelectron transit layer, the damage is removed. More specifically, thethermal oxide film is formed from the damaged surface toward the insideof the electron transit layer, and consequently, the interface betweenthe thermal oxide film and the electron transit layer is located in aregion which is not damaged. Then, since a channel is formed on theinterface that is not damaged, the electron mobility in the channelbecomes an original value of the nitride semiconductor forming theelectron transit layer.

As described above, the recess formed in the electron supply layer ismade to reach the electron transit layer, and thus it is possible toreliably achieve a normally off-type, and it is possible to provide thenitride semiconductor device of the HEMT structure where the electronmobility is high in the channel.

As in the case of the preferred embodiment of A1, the combination of theelectron supply layer/electron transit layer may be any one of AlGaNlayer/GaN layer, AlGaN layer/AlGaN layer (where Al composition isdifferent), AlInN layer/AlGaN layer, AlInN layer/GaN layer, AlNlayer/GaN layer and AlN layer/AlGaN layer. More generally, the electronsupply layer may contain Al and N in its composition. The electrontransit layer may contain Ga and N in its composition, and thecomposition (in particular, Al composition) is different from theelectron supply layer. The electron supply layer and the electrontransit layer are different in composition (in particular, Alcomposition), and thus a lattice mismatch occurs therebetween, with theresult that a two-dimensional electron gas caused by polarization isproduced within the electron transit layer near the interface.

A11. The relative permittivity of the gate insulating film is preferablyhigher than the relative permittivity of the electron supply layer. Inthis way, it is possible to generate a sufficient electric field toinduce a channel by applying a bias to the gate electrode.

A12. The gate insulating film is preferably an insulating film that isformed by an ALD method. Since it is possible to control the filmthickness of the gate insulating film at the atomic level by the ALD(Atomic Layer Deposition) method, it is possible to provide a nitridesemiconductor device in which the device properties are accuratelycontrolled.

A13. The gate insulating film may be formed of Al₂O₃. Since Al₂O₃ is aninsulating film that can be formed by the ALD method, it is possible toprovide a nitride semiconductor device in which the device propertiesare accurately controlled by using Al₂O₃ to form the gate insulatingfilm.

A14. At a lowermost part of the recess, approximately half of the filmthickness of the gate insulating film may be located on the side of theelectron transit layer with respect to an interface between the electrontransit layer and the electron supply layer. In this configuration, thebottom portion of the recess is made to reliably reach the electrontransit layer, and thus it is possible to achieve a normally off-typedevice.

A15. The film thickness of the gate insulating film may be 5 to 50 nm.In this way, it is possible to acquire sufficient voltage resistancebetween the gate and the source while reducing the on-resistance. Inother words, when the gate insulating film is excessively thin, thevoltage resistance is insufficient whereas when the gate insulating filmis excessively thick, the on-resistance is increased.

A16. The film thickness of the electron transit layer is preferably 5 to2000 nm. When the thickness of the electron transit layer exceeds 2000nm, a crack is more likely to be produced. When the thickness of theelectron transit layer is less than 50 nm, the mobility is lowered. Bycontrast, when the thickness of the electron transit layer is reduced,the threshold value (the lowest voltage that needs to be applied to thegate electrode to establish electrical continuity between the source andthe drain) can be increased, and a circuit design for driving the deviceis easily performed. In other words, when the thickness of the electrontransit layer is increased to enhance the mobility, the threshold valueis lowered, and thus a circuit design becomes difficult. In thispreferred embodiment, the surface of the electron transit layer isthermally oxidized, and thus its surface is prevented from beingconverted into an n-type, with the result that it is possible toincrease the threshold value. Hence, while the thickness of the electrontransit layer is relatively increased to acquire a high electronmobility, it is possible to provide a device having a high thresholdvalue.

A17. The electron transit layer may be formed of GaN, and the electronsupply layer may be formed of AlGaN.

A18. According to this preferred embodiment, there is provided a methodof manufacturing a nitride semiconductor device, the method including: astep of forming an electron transit layer that is formed of a nitridesemiconductor; a step of forming, on the electron transit layer, anelectron supply layer that is formed of a nitride semiconductor whosecomposition is different from the electron transit layer; a step offorming a recess by etching the electron supply layer until the electrontransit layer is exposed; a step of thermally oxidizing a surface of theelectron transit layer exposed at the recess; a step of forming a gateinsulating film by embedding an insulating material within the recess; astep of forming a gate electrode that is opposite to the electrontransit layer across the gate insulating film within the recess; and astep of forming a source electrode and a drain electrode that are incontact with the electron supply layer in a position spaced apart withinthe gate electrode intervening therebetween.

In this method, a structure where the gate electrode is opposite to theelectron transit layer via the gate insulating film embedded within therecess formed in the electron supply layer is provided, and thus it ispossible to manufacture a nitride semiconductor device having a normallyoff-type HEMT structure. Since after the recess is formed by etching,the surface of the electron transit layer exposed within the recess isthermally oxidized, the damage on the surface of the electron transitlayer by etching is removed. In this way, since the electron mobility inthe channel region immediately below the recess becomes an original orinherent value of the nitride semiconductor forming the electron transitlayer, it is possible to realize excellent device properties.

A19. In the thermally oxidizing step, a thermal oxide film may be formedon the surface of the electron transit layer exposed within the recess.

A20. In the thermally oxidizing step, a thermal oxide film may be formedon surfaces of the electron transit layer and the electron supply layerexposed within the recess.

A21. The thermal oxide layer may contain Ga or O. When the electrontransit layer has a composition including Ga, the thermal oxide filmcontains Ga and O.

A22. The etching that forms the recess may be dry etching using anetching agent including at least one type of element of B, Cl and Si.

A23. The concentrations of B, Cl and Si in the surface of the gateinsulating film on the side of the electron transit layer are preferably10²⁰ cm⁻³ or less. Since B, Cl and Si included in an etching agent reactwith oxygen at the time of thermal oxidization and are scattered, theirconcentrations left on the surface of the gate insulating film on theside of the electron transit layer are not high, and are 10²⁰ cm⁻³ orless.

A24. The step of forming the gate insulating film is preferably a stepof forming the insulating material by an ALD method. In this way, sinceit is possible to control the film thickness of the gate insulating filmat the atomic level, it is possible to accurately control the deviceproperties of the nitride semiconductor device.

A25. The insulating material may be Al₂O₃. Since Al₂O₃ can be formed bythe ALD method, it is possible to accurately control the deviceproperties of the nitride semiconductor device.

This preferred embodiment will be described in detail below withreference to the accompanying drawings.

FIG. 1 is a schematic cross-sectional view for illustrating theconfiguration of a nitride semiconductor device according to a preferredembodiment of the present invention. FIG. 2 is a plan view of thenitride semiconductor device described above. A cross section takenalong line I-I in FIG. 2 is shown in FIG. 1.

This nitride semiconductor device includes a substrate 101 (for example,a silicon substrate), a buffer layer 102 that is formed on the surfaceof the substrate 101, an electron transit layer 103 that is epitaxiallygrown on the buffer layer 102 and an electron supply layer 104 that isepitaxially grown on the electron transit layer 103. The nitridesemiconductor device further includes a passivation film 105 that coversthe surface of the electron supply layer 104, and a source electrode 106and a drain electrode 107 that pass through contact holes 106 a and 107a formed in the passivation film 105 to make ohmic contact with theelectron supply layer 104. The source electrode 106 and the drainelectrode 107 are arranged at an interval, and a gate electrode 108 isarranged therebetween.

In the electron supply layer 104, a recess 109 that is dug from itssurface toward the electron transit layer 103 is formed. FIG. 3 shows anenlarged view of an example of a structure in the vicinity of the recess109 in an actual element. The recess 109 is formed such that its depthreaches the electron transit layer 103. Hence, both the electron transitlayer 103 and the electron supply layer 104 are opposite to the innerspace of the recess 109. In the inner wall surface of the recess 109, apart on the side of the electron transit layer 103 with respect to theinterface between the electron transit layer 103 and the electron supplylayer 104 is referred to as a “bottom portion 109 a,” and a part on theside of the electron supply layer 104 with respect to the interface isreferred to as a “side wall portion 109 b.” The recess 109 passesbetween the source electrode 106 and the drain electrode 107 and isformed so as to be groove-shaped, and may have, as shown in FIG. 3, asurface whose cross section is formed in the shape of a smoothlyrecessed dish or a parabola. For example, the lowermost part of thebottom portion 109 a of the recess 109 is located on the side of theelectron transit layer 103 with respect to the interface between theelectron transit layer 103 and the electron supply layer 104. Thelowermost part of the bottom portion 109 a of the recess 109 may belocated on the side of the electron transit layer 103, 10 nm to 100 nmand preferably 10 nm to 20 nm from the interface between the electrontransit layer 103 and the electron supply layer 104.

On the bottom portion 109 a and the side wall portion 109 b of therecess 109, a thermal oxide film 111 is formed. A gate insulating film110 is laminated on the thermal oxide film 111. At the bottom portion109 a of the recess 109, the gate electrode 108 is opposite to theelectron transit layer 310 across the gate insulating film 110 and thethermal oxide film 111. The thermal oxide film 111 continuously extendsnot only to the bottom portion 109 a of the recess 109 but also to theside wall portion 109 b, and thus it is possible to reduce a leak pass.

The electron transit layer 103 and the electron supply layer 104 areformed of a group-III nitride semiconductor (hereinafter simply referredto as a “nitride semiconductor”) having a different composition. Forexample, the electron transit layer 103 may be formed with a GaN layer,and its thickness may be 50 to 2000 nm. In this preferred embodiment,the electron supply layer 104 is formed with an Al_(X)G_(1-X)N layer(0<x<1), and its thickness may be, for example, about 25 nm.

As described above, the electron transit layer 103 and the electronsupply layer 104 are formed of a nitride semiconductor having adifferent composition to form a heterojunction, and a lattice mismatchoccurs therebetween. Due to polarization caused by the heterojunctionand the lattice mismatch, in a position (for example, a position of adistance of about a few angstroms from the interface) near the interfacebetween the electron transit layer 103 and the electron supply layer104, a two-dimensional electron gas 115 caused by the polarization isspread.

On the bottom portion 109 a of the recess 109, the thermal oxide film111 has a film thickness of 1 to 100 nm, preferably 1 to 5 nm and morepreferably 1 to 3 nm. The thermal oxide film 111 is formed by thermallyoxidizing the surface of the electron transit layer 103, and is thus incontact with the electron transit layer 103. The interface between thethermal oxide film 111 and the electron transit layer 103 is located ata depth on the electron transit layer 103 side with respect to theinterface between the electron supply layer 104 and the electron transitlayer 103. The thermal oxide film 111 may have a surface part which islocated at a depth on the electron transit layer 103 side with respectto the interface between the electron supply layer 104 and the electrontransit layer 103 on the bottom portion 109 a of the recess 109.

The thermal oxide film 111 includes a bottom portion covering portion111 a (a first part of the thermal oxide film 111) that is formed bythermally oxidizing the electron transit layer 103 exposed at the bottomportion 109 a of the recess 109, and a side wall covering portion 111 b(a second part of the thermal oxide film 111) that is formed bythermally oxidizing the surface of the electron supply layer 104 exposedat the side wall portion 109 b of the recess 109. Since the bottomportion covering portion 111 a is a part that is formed by thermallyoxidizing the exposed surface of the electron transit layer 103 made ofGaN, its composition includes Ga and O. Since the side wall coveringportion 111 b is a part that is formed by thermally oxidizing theexposed surface of the electron supply layer 104 made of AlGaN, itscomposition also includes Ga and O. The bottom portion covering portion111 a and the side wall covering portion 111 b are formed by a commonthermal oxidizing step, and are continuous with each other. Since GaNand AlGaN have a different thermal oxidation rate, the bottom portioncovering portion 111 a and the side wall covering portion 111 b differin film thickness accordingly. Specifically, the film thickness of thebottom portion covering portion 111 a is larger than that of the sidewall covering portion 111 b. The film thickness of the bottom portioncovering portion 111 a is 1 to 100 nm, preferably 1 to 5 nm and morepreferably 1 to 3 nm. The film thickness of the side wall coveringportion 111 b is likewise 1 to 100 nm, preferably 1 to 5 nm and morepreferably 1 to 3 nm; however, it is smaller than that of the bottomportion covering portion 111 a.

In the thermal oxide film 111 formed by thermal oxidization from theexposed surface, an oxygen concentration in the film has a gradient withrespect to the direction of the film thickness. More specifically, theoxygen concentration in the thermal oxide film 111 is maximum at theinterface with the gate insulating film 110, and is decreased as itapproaches toward the electron transit layer 103. The maximum oxygenconcentration in the thermal oxide film 111 may be equal to or less than10²⁰ cm⁻³.

The gate insulating film 110 is an insulating film whose relativepermittivity is higher than that of the electron supply layer 104. Thegate insulating film 110 may be an insulating film that is formed by anALD (Atomic Layer Deposition) method. More specifically, the gateinsulating film 110 is formed of, for example, Al₂O₃ (aluminum oxide,alumina), and is formed to be thicker than the thermal oxide film 111(for example, may be thicker than the electron supply layer 104). Morespecifically, the film thickness of the gate insulating film 110 ispreferably 5 to 50 nm (for example, 20 nm). In this way, it is possibleto obtain a necessary breakdown voltage (for example, 20 V or more) andto reduce an on-resistance. The film thickness of the gate insulatingfilm 110 may be about twice as large as that of the thermal oxide film111. In the gate insulating film 110, at the lowermost part of thebottom portion 109 a of the recess 109, about half of the film thicknessmay be located on the side of the electron transit layer 103 withrespect to the interface between the electron transit layer 103 and theelectron supply layer 104.

In this preferred embodiment, in the surface of the gate insulating film110 on the opposite side to the gate electrode 108, that is, theinterface with the thermal oxide film 111, the concentration of each ofB, Cl and Si is equal to or less than 10²⁰ cm⁻³.

In this preferred embodiment, the gate insulating film 110 is in contactwith the thermal oxide film 111 within the recess 109, further extendsto the outside of the recess 109 and covers the surface of the outsideof the recess 109 of the electron supply layer 104. In this way, thevoltage resistance is further enhanced.

The gate electrode 108 is formed so as to be in contact with the gateinsulating film 110. The gate electrode 108 may be formed with amultilayer electrode film having a lower layer in contact with the gateinsulating film 110 and an upper layer stacked on the lower layer. Thelower layer may be formed of Ni or Pt, and the upper layer may be formedof Au or Al. A gate electrode 8 is displaced to a source electrode 6,and thereby has a non-symmetrical structure in which a gate to draindistance (for example, 9 μm) is longer than a gate to source distance(for example, 2 μm). The non-symmetrical structure alleviates a highelectric field produced between the gate and the drain to facilitate theenhancement of the voltage resistance. The gate length may be about 1μm, and the gate width may be about 175 μm.

Furthermore, in this preferred embodiment, the gate electrode 108includes a gate main body portion 181 that enters an opening 105 aformed in the passivation film 105 and that further enters the recess109, and a field plate portion 182 that is continuous with the gate mainbody portion 181 and that extends, outside the opening 105 a, toward thedrain electrode 107 on the passivation film 105. More specifically, thepart opposite to the bottom portion 109 a of the recess 109 in thedirection of the thickness of the electron transit layer 103 is the gatemain body portion 181, and the part thereoutside (in particular, thepart extending toward the side of the drain electrode 107) is the fieldplate portion 182. A distance Lfp (for example, about 2.25 μm) from adrain end 181 a that is an end portion of the lower end of the gate mainbody portion 181 on the side of the drain electrode 107 to an endportion of the field plate portion 182 on the side of the drainelectrode 107 refers to a field plate length. Specifically, the drainend 181 a is defined by the position on the side of the drain electrode107 where the surface (the interface between the thermal oxide film 111and the gate insulating film 110) of the recess 109 crosses a planeincluding the interface between the electron transit layer 103 and theelectron supply layer 104.

The field plate length Lfp is preferably equal to or more than one-sixtha distance Lgd (for example, about 9 μm) from the drain end 181 a to thedrain electrode 107 but equal to or less than half of the distance Lgd.In this way, it is possible to alleviate electric field concentration atthe drain end 181 a and to prevent the passivation film 105 from beingbroken due to the electric field between the drain side end of the fieldplate portion 182 and the drain electrode 107.

The source electrode 106 and the drain electrode 107 are an ohmicelectrode that includes, for example, Ti, Al, Mo and Si, and are inohmic contact with the two-dimensional electron gas 115. The ohmicelectrode may be an electrode obtained by forming a multilayer metalfilm on the electron supply layer 104, patterning the multilayer metalfilm and then performing sintering processing thereon. The multilayermetal film may be formed by depositing, on the electron supply layer104, a Ti layer (for example, 200 angstroms in thickness), a first Silayer (for example, 200 angstroms in thickness), an Al layer (forexample, 2000 angstroms in thickness), a second Si layer (for example,200 angstroms in thickness) and a Mo layer (for example, 2000 angstromsin thickness) in this order. In this case, the sintering processing ispreferably performed at a temperature at which the Al layer is melted.

The buffer layer 102 may be, for example, an AlGaN layer or may be alayer that has a superlattice structure in which an AlN layer and a GaNlayer are repeatedly deposited in layers.

In this nitride semiconductor device, on the electron transit layer 103,the electron supply layer 104 having a composition different from theelectron transit layer 103 is formed, and thus the heterojunction isformed. In this way, within the electron transit layer 103 in thevicinity of the interface between the electron transit layer 103 and theelectron supply layer 104, the two-dimensional electron gas 115 isformed, and a HEMT that utilizes the two-dimensional electron gas 115 asa channel is formed. The gate electrode 108 is opposite to the electrontransit layer 103 across the gate insulating film 110, and the electronsupply layer 104 is not present immediately below the gate electrode108. Hence, immediately below the gate electrode 108, thetwo-dimensional electron gas 115 resulting from polarization caused bythe lattice mismatch between the electron supply layer 104 and theelectron transit layer 103 is not formed. Thus, when no bias is appliedto the gate electrode 108 (at the time of zero bias), the channelresulting from the two-dimensional electron gas 115 is interruptedimmediately below the gate electrode 108. In this way, a normallyoff-type HEMT is realized. When an appropriate on-voltage (for example,5 V) is applied to the gate electrode 108, a channel is induced withinthe electron transit layer 103 immediately below the gate electrode 108,and the two-dimensional electron gases 115 on both sides of the gateelectrode 108 are connected. In this way, electrical continuity isestablished between the source and the drain.

In use, for example, between the source electrode 106 and the drainelectrode 107, a predetermined voltage (for example, 200 to 400 V) isapplied such that the side of the drain electrode 107 is positive. Inthis state, an off-voltage (0 V) or an on-voltage (5 V) is applied tothe gate electrode 108 with the source electrode 106 being in areference potential (0 V).

As shown in FIG. 2, in a plan view, the gate electrode 108 is routedsuch that a junction region (source junction region, a region within thecontact hole 106 a) Sa between the source electrode 106 and the electronsupply layer 104 is separated from a junction region (drain junctionregion, a region within the contact hole 107 a) Da between the drainelectrode 107 and the electron supply layer 104. More specifically, ajunction region (gate junction region, a region within the opening 105a) Ga between the gate main body portion 181 of the gate electrode 108and the electron supply layer 104 is formed as a belt-shaped patternhaving a constant width that separates the source junction region Sa andthe drain junction region Da. More specifically, the source junctionregion Sa and the drain junction region Da are rectangular regions inwhich the longitudinal directions thereof are parallel to each other,and are aligned along the lateral direction of the rectangular region.The gate junction region Ga is formed in a zigzag-shape that passesbetween the source junction region Sa and the drain junction region Da.The gate junction region Ga is arranged so as to pass through a positionclose to the source junction region Sa as compared with the drainjunction region Da. A distance between the gate junction region Ga andan edge of the gate electrode 108 on the side of the drain junctionregion Da is a field plate length Lfp. The width of the gate junctionregion Ga is a gate length Lg (for example, about 1 μm).

FIGS. 4A to 4F are schematic cross-sectional views for illustrating anexample of a step of manufacturing the nitride semiconductor device, andshow cross-sectional structures at a plurality of stages in themanufacturing step. FIGS. 5A to 5C are enlarged cross-sectional viewsshowing an example of structures halfway through the manufacturing stepin the vicinity of the recess 109 in an actual element.

First, as shown in FIG. 4A, on the substrate 101, the buffer layer 102and the electron transit layer 103 are sequentially epitaxially grown,and furthermore, on the electron transit layer 103, the electron supplylayer 104 is epitaxially grown. Then, furthermore, the passivation film105 is formed by, for example, a CVD method (chemical vapor depositionmethod) such that the entire surface on the electron supply layer 104 iscoated. The passivation film 105 may be formed of silicon nitride (SiN),and it is appropriate that its film thickness is about a few hundrednanometers.

Next, as shown in FIGS. 4B and 5A, the recess 109 is formed in theelectron supply layer 104 so as to correspond to a position where thegate electrode 108 is formed. Specifically, a mask having an opening isformed in a position where the recess 109 is to be formed, thepassivation film 105 is opened by dry etching via the mask and etchingis further performed until the electron supply layer 104 reaches theelectron transit layer 103, with the result that the recessed recess 109is formed from the surface of the electron supply layer 104 toward theelectron transit layer 103. Here, the electron transit layer 103 isexposed to the bottom portion 109 a of the recess 109, and the electronsupply layer 104 is exposed to the side wall portion 109 b. The surfaceof the bottom portion 109 a of the recess 109 is located on the side ofthe electron transit layer 103 with respect to the interface between theelectron transit layer 103 and the electron supply layer 104. The dryetching may be plasma etching using a BCl₃ gas or a SiCl₄ gas as anetching gas.

Next, as shown in FIGS. 4C and 5B, the thermal oxide film 111 is formedby thermal oxidization processing (selective thermal oxidizingprocessing) that is performed while an oxygen gas is passed within athermal oxidation furnace. More specifically, in an atmospherecontaining a nitrogen gas and an oxygen gas, thermal processing isperformed at 900° for 3 minutes, and thus the inner wall surface of therecess 109 is selectively thermally oxidized, with the result that thethermal oxide film 111 is formed on the inner wall surface of the recess109. In this way, the thermal oxide film 111 having the bottom portioncovering portion 111 a on the exposed surface of the electron transitlayer 103 within the recess 109 and the side wall covering portion 111 bon the exposed surface of the electron supply layer 104 within therecess 109 is formed. The thermal oxide film 111 is formed by oxidizing,from the surfaces of the electron transit layer 103 and the electronsupply layer 104 toward the inside, the nitride semiconductor materialsthereof. Here, since the position of the inner wall surface of therecess 109 is hardly changed, the surfaces of the bottom portioncovering portion 111 a and the side wall covering portion 111 b on theside of the recess 109 are continuous. On the other hand, since thecompositions of the electron transit layer 103 and the electron supplylayer 104 are different, their oxidization rates are differentaccordingly. Hence, the bottom portion covering portion 111 a and theside wall covering portion 111 b differ from each other in thickness,and the bottom portion covering portion 111 a is thicker than the sidewall covering portion 111 b. Thus, in a boundary portion between thebottom portion covering portion 111 a and the side wall covering portion111 b, a step is produced on the surface on the opposite side to therecess 109.

Next, as shown in FIGS. 4D and 5C, the gate insulating film 110 isformed so as to cover the entire exposed surface. Hence, the gateinsulating film 110 is formed to make contact with the thermal oxidefilm 111 within the recess 109 and to extend to a region outside therecess 109. In this preferred embodiment, the gate insulating film 110is formed of alumina (Al₂O₃), and is formed by, for example, an ALDmethod.

Next, as shown in FIG. 4E, the source electrode 106 and the drainelectrode 107 are formed. Specifically, the contact holes 106 a and 107a passing through the gate insulating film 110 and the passivation film105 are formed so as to correspond to the positions where the electrodes106 and 107 are to be formed, and then the source electrode 106 and thedrain electrode 107 are formed. The source electrode 106 and the drainelectrode 107 are ohmic electrodes that are in ohmic contact with thetwo-dimensional electron gas 115 (see FIG. 1).

The step of forming the ohmic electrode includes the step of forming themultilayer metal film by sequentially depositing, for example, on theelectron supply layer 104, the Ti layer (for example, 200 angstroms inthickness), the first Si layer (for example, 200 angstroms inthickness), the Al layer (for example, 2000 angstroms in thickness), thesecond Si layer (for example, 200 angstroms in thickness) and the Molayer (for example, 2000 angstroms in thickness). The multilayer metalfilm is formed by sequentially evaporating or sputtering the constituentlayers. Thereafter, a multilayer electrode film is patterned on thepattern of the source electrode 106 and the drain electrode 107. Thepatterning of the multilayer electrode film may be performed by liftingoff or may be performed by etching. After the patterning, furthermore,sintering processing is performed, and thus the source electrode 106 andthe drain electrode 107 in ohmic contact with the two-dimensionalelectron gas 115 are formed. The sintering processing is preferablyperformed such that the Al layer is melted, and is performed at atemperature (for example, 850° C.) higher than the melting point (565°C.) of Al. Specifically, the sintering processing is preferablyperformed at 850° C. for about 35 minutes.

Next, as shown in FIG. 4F, a resist film 116 having an opening in aposition where the gate electrode 108 is to be formed is formed, and inthis state, an electrode film 117 is formed so as to cover the entiresurface. The opening of the resist film 116 includes a region of theopening 105 a formed in the passivation film 105, and is formed in aregion wider than the region of the opening 105 a. The edge portion ofthe opening of the resist film 116 on the side of the drain electrode107 is placed backward by the field plate length Lfp from the drain sideend of the opening 105 a of the passivation film 105 (more exactly, theposition where the surface of the recess 109 crosses, on the drain side,the interface between the electron transit layer 103 and the electronsupply layer 104) toward the drain electrode 107. The electrode film 117is, for example, formed with a multilayer metal film obtained bydepositing a lower layer made of Ni or Pt and an upper layer made of Auor Al, and is formed by sequentially evaporating the layers.

Next, together with the resist film 116, the electrode film 117 (anunnecessary part of the electrode film 117) on the resist film 116 islifted off, and thus the electrode film 117 is patterned, with theresult that the gate electrode 108 is obtained. In this way, the nitridesemiconductor device having the structure shown in FIG. 1 can beobtained. Thereafter, the entire surface is covered with an interlayerinsulating film, and contact holes for exposing the source electrode 106and the drain electrode 107 are formed in the interlayer insulatingfilm. Then, on the interlayer insulating film, a source wiring and adrain wiring are formed such that they are respectively connected withthe source electrode 106 and the drain electrode 107 through the contactholes.

As described above, in this preferred embodiment, the thermal oxide film111 is formed in the surface of the electron transit layer 103 exposedwithin the recess 109, and the gate insulating film 110 is in contactwith the thermal oxide film 111. When etching for forming the recess 109is performed, damage is given to the surface of the electron transitlayer 103 at the bottom portion of the recess 109. This damage isremoved in the thermal oxidization process for forming the thermal oxidefilm 111. More specifically, the thermal oxide film 111 is formed fromthe damaged surface toward the inside of the electron transit layer 103,with the result that the interface between the thermal oxide film 111and the electron transit layer 103 is located in a region which is notdamaged. Then, since a channel is formed in the interface that is notdamaged, the electron mobility of the channel becomes an original orinherent value of the nitride semiconductor forming the electron transitlayer 103. In this way, the recess 109 forming the electron supply layer104 is made to reach the electron transit layer 103, and thus it ispossible to reliably achieve a normally off-type and it is also possibleto realize a HEMT structure in which the electron mobility in thechannel is high.

In the configuration of this preferred embodiment, the exposed surfaceof the electron transit layer 103 within the recess 109 is selectivelythermally oxidized, and thus it is possible to prevent the surface frombeing converted into an n-type, with the result that the threshold valuecan be increased. Hence, while the thickness (for example, 50 to 200 nm)of the electron transit layer 103 is relatively increased to acquire ahigh electron mobility, it is possible to provide the device having ahigh threshold value.

FIG. 6A is a characteristic diagram obtained by examining a relationship(Ids-Vds characteristics) between a drain current Ids and a drainvoltage Vds for various gate voltages Vgs in the nitride semiconductordevice having the structure described above. FIG. 6B shows transfercharacteristics, and FIG. 6C shows voltage resistance characteristics(3-terminal off-state characteristics) in an off-state. A thresholdvoltage Vth is about 1.5 V and is a sufficiently high voltage value. Themaximum value of the drain current Ids is 150 mA/mm, the maximum valueof a mutual conductance gm is 45 mS/mm, the on-resistance Ron is 33Ω·mm, and a voltage resistance V_(BR) (breakdown voltage between thesource and the drain in an off-state) is higher than 100 V.

FIG. 7 shows variations in the threshold value Vth for a plurality ofsamples of the nitride semiconductor device. In the samples (withthermal oxidization) of the nitride semiconductor device in theconfiguration of the present preferred embodiment, the threshold valueVth is 1.5 V or more, and the standard deviation σ indicating variationsin the threshold value Vth is 0.06 V. By contrast, in a plurality ofsamples in a comparative example (w/o thermal oxidization) whoseconfiguration is the same as in the preferred embodiment described aboveexcept that the thermal oxidization processing (the formation of thethermal oxide film 111) after the recess was not performed, thethreshold value Vth is often less than 1.5 V, significant variations inthe threshold value Vth are produced and the standard deviation σ of thethreshold value Vth is about 0.2V. Hence, it is found that in theconfiguration of this preferred embodiment, it is possible to realizeand stabilize a high threshold value Vth.

Variations of this preferred embodiment are possible as follows. Forexample, although in the preferred embodiment described above, theexample where the electron transit layer 103 is formed with the GaNlayer and the electron supply layer 104 is formed of AlGaN has beendescribed, any other combination can be performed as long as thecompositions (in particular, Al composition) of the electron transitlayer 103 and the electron supply layer 104 are different. For example,examples of the combination of the electron transit layer 103/theelectron supply layer 104 can include GaN/AlGaN,Al_(m)Ga_(1-m)N/Al_(n)Ga_(1-n)N (where m≠n), AlGaN/AlInN, GaN/AlInN,GaN/AlN and AlGaN/AlN.

Although in the preferred embodiment described above, the gateinsulating film 110 is formed with one insulating film made of Al₂O₃,the gate insulating film 110 may be formed by depositing two or moreinsulating films to further enhance breakdown voltage resistance.

Although in the preferred embodiment described above, as an example ofthe material of the substrate 1, silicon is indicated, any substratematerial such as a sapphire substrate or a GaN substrate can be applied.

The features of a semiconductor device according to a second preferredembodiment of the present invention are as follows.

B1. According to this preferred embodiment, there is provided a nitridesemiconductor device including: a substrate; an electron transit layerthat is formed on the substrate, and that is formed of a nitridesemiconductor; an electron supply layer that is formed on the electrontransit layer, and that is formed of a nitride semiconductor whosecomposition is different from the electron transit layer; an AlGaNbuffer layer that intervenes between the substrate and the electrontransit layer, and that includes a high aluminum composition regionwhose aluminum composition is relatively high and a low aluminumcomposition region whose aluminum composition is lower than the highaluminum composition region and which is arranged in a region close tothe electron transit layer as compared with the high aluminumcomposition region; and an element separation layer that is formed witha region whose resistance is increased by causing a crystal defectthrough ion implantation, and that passes through the electron supplylayer and the electron transit layer to reach the AlGaN buffer layer.

In this configuration, the electron transit layer and the electronsupply layer formed on the substrate are formed of nitridesemiconductors having different compositions. Hence, a heterojunction isformed therebetween. In this way, in the vicinity of the interfacebetween the electron transit layer and the electron supply layer, thetwo-dimensional electron gas is formed, and thus it is possible to forma device that utilizes the two-dimensional electron gas and that has ahigh mobility.

On the other hand, between the substrate and the electron transit layer,the AlGaN buffer layer is formed. Furthermore, the element separationlayer passing through the electron supply layer and the electron transitlayer reaches the AlGaN buffer layer. The element separation layer isformed with a region whose resistance is increased by causing a crystaldefect in the nitride semiconductor by ion implantation. With theelement separation layer, it is possible to perform element separationin the direction along the direction of the major surface of thesubstrate.

The AlGaN buffer layer includes a high aluminum composition region and alow aluminum composition region, and the low aluminum composition regionis arranged in a region closer to the electron transit layer than thehigh aluminum composition region. Between the low aluminum compositionregion and the substrate, the high aluminum composition regionintervenes, and thus variations in energy level in the direction of thethickness of the AlGaN buffer layer are reduced. Hence, since thethickness of the potential barrier is increased, it is possible toreduce the leak caused by a quantum tunneling effect. In this way, it ispossible to interrupt the leak path via the buffer layer. In theconfiguration of Patent Document 1, the leak current of the elementformed in a semiconductor operation region cannot be sufficientlyreduced.

B2. The AlGaN buffer layer may be an AlGaN layer whose aluminumcomposition is adjusted such that as the AlGaN buffer layer approachesthe electron transit layer in the direction of a layer thickness fromthe substrate toward the electron transit layer, the aluminumcomposition is decreased. The aluminum composition in the direction ofthe layer thickness may be stepwise changed or continuously changed.

B3. The AlGaN buffer layer may include a first aluminum compositionAlGaN layer that has a first aluminum composition and a second aluminumcomposition AlGaN layer that is deposited on a side of the electrontransit layer with respect to the first aluminum composition AlGaN layerand that has a second aluminum composition lower than the first aluminumcomposition, and in this case, the high aluminum composition region mayinclude the first aluminum composition AlGaN layer, and the low aluminumcomposition region may include the second aluminum composition AlGaNlayer.

B4. The nitride semiconductor device may further include an AlN bufferlayer that intervenes between the AlGaN buffer layer and the substrate.The AlN buffer layer intervenes, and thus it is possible to prevent Gain the AlGaN buffer layer from damaging the substrate (for example,silicon substrate). When the feature of item B4 is combined with thefeature of item B3, a buffer layer is formed that has a structure wherethe AlN buffer layer, the first aluminum composition AlGaN layer and thesecond aluminum composition AlGaN layer are arranged sequentially fromthe substrate toward the electron transit layer. In this structure,between the AlN buffer layer and the second aluminum composition AlGaNlayer of a relatively small aluminum composition, the first aluminumcomposition AlGaN layer of a relatively large aluminum compositionintervenes. In this way, in the buffer layer, a trapezoidal potentialbarrier is formed in the direction of the thickness. Thus, it ispossible to effectively reduce the leak current.

As described in B5, the element separation layer is preferably formed soas to surround an element region. In this way, since the element regionis surrounded by the element separation layer, it is possible to reduceor prevent the leak between the inside and the outside of the elementregion.

B6. The nitride semiconductor device may further include a wiring thatis arranged on the element separation layer. In this configuration,since the space of the element separation layer can be utilized toarrange the wiring, the configuration is advantageous in highintegration. Moreover, since the element separation layer below thewiring is a high resistance layer thick enough to reach the AlGaN bufferlayer, it is possible to reduce the parasitic capacitance, with theresult that it is possible to perform a high-speed operationaccordingly. Hence, while the leak in each of the element regions isbeing reduced, it is possible to provide the nitride semiconductordevice capable of performing a high-speed operation.

B7. The element separation layer may be formed so as to separate aplurality of element regions, and an element-to-element wiring thatconnects between a plurality of elements formed in the element regionsmay be provided. In this configuration, while the leak in each of theelement regions is being reduced, a plurality of elements respectivelyformed in a plurality of element regions are coupled, with the resultthat it is possible to form a large element which realizes a desiredfunction.

B8. The elements respectively formed in the element regions may includetwo or more elements having different functions. In this way, while theleak in each of the element regions is being reduced, the elements ofdifferent functions are connected to each other, with the result that itis possible to form a large element which realizes a desired function.

B9. The elements respectively formed in the element regions may includetwo or more elements having a common function. In this way, while theleak in each of the element regions is being reduced, the elements of acommon function are connected to each other, with the result that it ispossible to form a large element which realizes a desired function (forexample, the desired current capacity).

A plurality of elements that are respectively formed in the elementregions may include a first element, a second element having the commonfunction to the first element and a third element having a differentfunction from the first element. In this way, by combining the elementof the common function and the element of the different function, it ispossible to form a large element having a desired function.

B10. The element separation layer may be formed so as to surround theelement regions of the elements connected with the element-to-elementwiring. In other words, the element separation layer may separate theindividual element regions and surround the element regions of aplurality of elements connected with the element-to-element wiring. Inthis way, it is possible to separate, from the surrounding area, a largeelement in which the elements are connected with the element-to-elementwiring. Thus, it is possible to further reduce the leak current.

B11. The element separation layer is preferably a high resistance layerthat is formed by ion implantation using, as a material, an elementwhose mass number is less than 10 but more than 2. The crystal structureof the nitride semiconductor is broken by ion implantation to have ahigh resistance, and with the high resistance layer formed in this way,it is possible to form the element separation layer. It is difficult foran ion species using an element of a large mass number as a material toreach the deep position of the nitride semiconductor layer. Hence, inorder to make an ion reach the AlGaN buffer layer, it is preferable touse an ion using, as a material, an element whose mass number is lessthan 10. On the other hand, an ion species using an element of a smallmass number as a material cannot produce a sufficient crystal defect inthe crystal structure of the nitride semiconductor, and for example, inthe thermal processing step performed in a stage following the ionimplantation, the crystal defect may be removed, with the result that itis likely that it is impossible to form a sufficiently large highresistance layer. Hence, in order to reliably increase the resistance ofthe nitride semiconductor layer, it is preferable to perform ionimplantation using an ion species using, as a material, an element whosemass number is more than 2. Consequently, the element separation layeris formed by ion implantation using an ion using, as a material, anelement whose mass number is more than 2 but less than 10, and thus itis possible to reduce the leak current.

B12. The element separation layer may be a high resistance layer that isformed by implantation of a helium ion. Since the helium ion is used,and thus it is possible to implant the ion into a deep position of thenitride semiconductor layer. For example, even when the electron transitlayer is thick, it is possible to form the element separation layerwhich reaches the buffer layer. In this way, it is possible to reducethe leak current.

B13. The element separation layer may be a high resistance layer that isformed by ion implantation using a plurality of acceleration energies.By the ion implantation (multistage ion implantation) using a pluralityof acceleration energy settings, it is possible to damage the nitridesemiconductor crystal from a shallow region to a deep region in thesurface on which the ion implantation is performed, with the result thatit is possible to form the element separation layer having a highresistance value regardless of the position of depth. In this way, it ispossible to provide the nitride semiconductor device in which the leakcurrent is reduced.

B14. The element separation layer may be a high resistance layer that isformed by ion implantation from a direction inclined with respect to adirection of thickness of the electron transit layer. Since ions aremade to easily collide with the atoms of the nitride semiconductorcrystal by the ion implantation from an inclined direction, it ispossible to accurately control the depth of the ion implantation. Inthis way, since the high resistance element separation layer is reliablyformed, it is possible to provide the nitride semiconductor device inwhich the leak current is reduced.

B15. An inclination angle of the ion implantation direction with respectto the direction of the thickness of the electron transit layer ispreferably 5 to 10 degrees. In this way, the depth of the ionimplantation is accurately controlled, and thus it is possible torealize a high resistance element separation layer, with the result thatit is possible to reduce the leak current.

B16. The electron transit layer is preferably formed of GaN, and theelectron supply layer is formed of AlGaN. In this way, the electrontransit layer and the electron supply layer have a heterojunction, andthe two-dimensional electron gas is formed on the side of the electrontransit layer in the vicinity of the interface therebetween. Thus, it ispossible to form a device utilizing the high mobility of the electronsof the two-dimensional electron gas.

Examples of the combination of the electron supply layer/electrontransit layer can include, in addition to AlGaN layer/GaN layer, AlGaNlayer/AlGaN layer (where Al composition is different), AlInN layer/AlGaNlayer, AlInN layer/GaN layer, AlN layer/GaN layer and AlN layer/AlGaNlayer. More generally, the electron supply layer may include Al and N inits composition. The electron transit layer may include Ga and N in itscomposition, and the Al composition may be different from the electronsupply layer. The electron supply layer and the electron transit layerare different in Al composition, and thus a lattice mismatch occurstherebetween, with the result that a carrier caused by polarizationcontributes to the formation of the two-dimensional electron gas.

B17. The electron transit layer is preferably a GaN layer having athickness of 400 nm or more. Since the electron transit layer formed ofGaN has a smooth and satisfactory surface state, it is possible toenhance the electron mobility. When the electron transit layer is a thinGaN layer, its surface state is not necessarily satisfactory, and it islikely that the mobility of the electrons of the two-dimensionalelectron gas formed in the vicinity of the GaN layer is affected by itssurface state and is thereby lowered.

B18. Preferably, the nitride semiconductor device further includes asource electrode and a drain electrode that are arranged at an intervalon the electron supply layer, and a gate electrode that is arrangedopposite to the electron transit layer between the source electrode andthe drain electrode. In this configuration, it is possible to controlthe two-dimensional electron gas immediately below the gate electrode byapplying a control voltage to the gate electrode, with the result thatit is possible to turn on and off between the source electrode and thedrain electrode and to control the current value therebetween. In thisway, it is possible to realize the HEMT (high-electron-mobilitytransistor) utilizing the high mobility of the electrons forming thetwo-dimensional electron gas.

B19. The gate electrode is preferably formed so as to surround thesource electrode together with the element separation layer. In thisway, an appropriate voltage is applied to the gate electrode, and thusit is possible to separate the two-dimensional electron gas between theside of the source electrode and the side of the drain electrode, withthe result that an interruption can be made between the source and thedrain.

B20. The nitride semiconductor device may further include an interlayerinsulating film that covers the source electrode, the drain electrodeand the gate electrode, a source wiring film that is connected to thesource electrode via a source contact hole passing through theinterlayer insulating film and that is arranged on the interlayerinsulating film, and a drain wiring film that is connected to the drainelectrode via a drain contact hole passing through the interlayerinsulating film and that is arranged on the interlayer insulating film.The source wiring film and the drain wiring film may be formed in such apattern that on the interlayer insulating film, the source wiring filmand the drain wiring film mesh with each other in a comb tooth shape.

In this configuration, a plurality of source electrodes and a pluralityof drain electrodes are alternately arranged in the shape of a stripe,and they can be respectively connected to the source wiring film and thedrain wiring film. Then, the gate electrode can be arranged between apair of the source electrode and the drain electrode adjacent to eachother. In this way, since the source electrode and the drain electrodeare opposite to each other via the gate electrode over a long range, thegate width (channel width) is increased, and thus it is possible toincrease the current.

B21. The gate wiring connected to the gate electrode may be arranged onthe element separation layer. In this way, since the gate electrode canbe arranged on a thick insulating layer including the element separationlayer, it is possible to reduce the capacity parasitic on the gatewiring. Thus, it is possible to provide the nitride semiconductor deviceof the HEMT structure where the leak current is reduced and a high-speedoperation can be performed.

The wiring arranged on the element separation layer is not limited tothe gate wiring connected to the gate electrode, and the source wiringconnected to the source electrode and the drain wiring connected to thedrain electrode may also be arranged on the element separation layer.

B22. There is provided a method of manufacturing a nitride semiconductordevice, the method including a step of forming an AlGaN buffer layer byepitaxially growing an AlGaN crystal on a substrate such that analuminum composition is relatively high in a region close to thesubstrate and is relatively low in a region away from the substrate, astep of forming an electron transit layer by epitaxially growing anitride semiconductor on the AlGaN buffer layer, a step of forming anelectron supply layer on the electron transit layer by epitaxiallygrowing a nitride semiconductor layer whose composition is differentfrom the electron transit layer and a step of forming a high resistanceelement separation layer that passes through the electron supply layerand the electron transit layer to reach the AlGaN buffer layer bybreaking a crystal structure through ion implantation on the electronsupply layer, the electron transit layer and the AlGaN buffer layer. Bythis method, it is possible to manufacture the nitride semiconductordevice of the structure described in item B1.

B23. The step of forming the AlGaN buffer layer may include a step offorming a first aluminum composition AlGaN layer of a first aluminumcomposition, and a step of forming, in a higher position than a positionof the first aluminum composition AlGaN layer, a second aluminumcomposition AlGaN layer of a second aluminum composition lower than thefirst aluminum composition. By this method, it is possible tomanufacture the nitride semiconductor device of the structure describedin item B3.

B24. The method may include a step of forming an AlN buffer layer on thesubstrate before the step of forming the AlGaN buffer layer such thatthe AlGaN buffer layer is formed on the AlN buffer layer. In this way,it is possible to manufacture the nitride semiconductor device of thestructure described in item B4.

B25. The element separation layer may be formed so as to surround anelement region. In this way, it is possible to manufacture the nitridesemiconductor device of the structure described in item B5.

B26. The element separation layer may be formed so as to separate aplurality of element regions, and the method of manufacturing a nitridesemiconductor device may further include a step of forming anelement-to-element wiring that connects between elements respectivelyformed in the element regions. In this way, it is possible tomanufacture the nitride semiconductor device of the structure describedin item B7.

B27. The ion implantation is preferably performed using, as a material,an element whose mass number is less than 10 but more than 2. In thisway, it is possible to manufacture the nitride semiconductor device ofthe structure described in item B11.

B28. The ion implantation is preferably performed using helium ions. Inthis way, it is possible to manufacture the nitride semiconductor deviceof the structure described in item B12.

B29. The ion implantation is preferably performed using a plurality ofacceleration energies. In this way, it is possible to manufacture thenitride semiconductor device of the structure described in item B13.

B30. The ion implantation is preferably performed from a directioninclined with respect to a major surface of the substrate. In this way,it is possible to manufacture the nitride semiconductor device of thestructure described in item B14.

B31. The ion implantation is preferably performed from a directioninclined at an angle of 5 to 10 degrees with respect to a normaldirection to the major surface of the substrate. In this way, it ispossible to manufacture the nitride semiconductor device of thestructure described in item B15.

B32. The step of forming the electron transit layer preferably includesa step of epitaxially growing a GaN layer having a thickness of 400 nmor more. In this way, it is possible to manufacture the nitridesemiconductor device of the structure described in item B17.

At least one of items B1 to B32 may be combined with at least one ofitems A1 to A25. In other words, it is possible to form a nitridesemiconductor device obtained by arbitrarily combining the features ofitems B1 to B32 with the features of items A1 to A25.

This preferred embodiment will be described in detail below withreference to the accompanying drawings.

FIG. 8 is a schematic cross-sectional view for illustrating theconfiguration of a nitride semiconductor device according to a preferredembodiment of the present invention.

This nitride semiconductor device includes a substrate 1 (for example, asilicon substrate), a buffer layer 2 that is formed on the surface ofthe substrate 1, an electron transit layer 3 that is epitaxially grownon the buffer layer 2, an electron supply layer 4 that is epitaxiallygrown on the electron transit layer 3 and a cap layer 5 that isepitaxially grown on the electron supply layer 4. This nitridesemiconductor device further includes a source electrode 6 and a drainelectrode 7 that are in ohmic contact with the electron supply layer 4via the cap layer 5. The source electrode 6 and the drain electrode 7are arranged at an interval in a direction parallel to a major surfaceof the electron supply layer 4, and the gate electrode 8 is arrangedtherebetween. The cap layer 5 is covered with the passivation film 9.

The passivation film 9 may be formed of silicon nitride (SiN), and it isappropriate that its film thickness is about a few hundred nanometers.In this preferred embodiment, the passivation film 9 has a two-layerstructure in which a lower layer 91 and an upper layer 92 are present.In the lower layer 91, contact holes 6 a and 7 a are formed, and thesource electrode 6 and the drain electrode 7 are respectively in contactwith the cap layer 5 via the contact holes 6 a and 7 a. Furthermore, inthe passivation film 9, an opening 8 a in which the gate electrode 8 isembedded is formed to pass therethrough. The cap layer 5 that is exposedto the bottom surface of the opening 8 a is covered with a gateinsulating film 10. In this preferred embodiment, the gate insulatingfilm 10 is continuously formed so as to cover not only the bottomsurface of the opening 8 a but also its side surface, and furthermore,the surface of the passivation film 9 outside the opening 8 a. The gateelectrode 8 is opposite to the cap layer 5 across the gate insulatingfilm 10.

The gate insulating film 10 is an insulating film whose relativepermittivity is higher than those of the electron supply layer 4 and thecap layer 5. The gate insulating film 10 may be an insulating film thatis formed by an ALD (Atomic Layer Deposition) method. More specifically,the gate insulating film 10 is formed of, for example, Al₂O₃ (aluminumoxide, alumina), and its film thickness is preferably 5 to 50 nm (forexample, 20 nm). In this way, it is possible to obtain a necessarybreakdown voltage (for example, 20 V or more) and to reduce anon-resistance.

The surfaces of the gate electrode 8 and the passivation film 9 arecovered with an interlayer insulating film 12. The interlayer insulatingfilm 12 is formed with, for example, a SiO₂ film whose film thickness isabout 1 μm. On the surface of the interlayer insulating film 12, asource wiring film 16 and a drain wiring film 17 are formed. The sourcewiring film 16 is connected to the source electrode 6 via a sourcecontact hole 16 a that is formed to pass through the interlayerinsulating film 12, the gate insulating film 10 and the upper layer 92of the passivation film 9. Likewise, the drain wiring film 17 isconnected to the drain electrode 7 via a drain contact hole 17 a that isformed to pass through the interlayer insulating film 12, the gateinsulating film 10 and the upper layer 92 of the passivation film 9.

The electron transit layer 3 and the electron supply layer 4 are formedof a group-III nitride semiconductor (hereinafter simply referred to asa “nitride semiconductor”) having a different composition. For example,the electron transit layer 3 may be formed with a GaN layer, and itsthickness may be about 400 nm to 1 μm. In this preferred embodiment, theelectron supply layer 4 is formed with an Al_(X)Ga_(1-X)N layer (0<x<1),and its thickness may be, for example, about 25 nm.

As described above, the electron transit layer 3 and the electron supplylayer 4 are formed of a nitride semiconductor having a differentcomposition to form a heterojunction, and a lattice mismatch occurstherebetween. Due to polarization caused by the heterojunction and thelattice mismatch, in a position (for example, a position of a distanceof about a few angstroms from the interface) within the electron transitlayer 3 near the interface between the electron transit layer 3 and theelectron supply layer 4, a two-dimensional electron gas 11 caused by thepolarization is spread.

The cap layer 5 is formed of GaN, which is a nitride semiconductor ofthe same composition as the electron transit layer 3, and its thicknessis 16 nm or less (more preferably 8 nm or less, and for example, about 3nm). The cap layer 5 contributes to the improvement of the surfacemorphology of the nitride semiconductor layer. In other words, since onthe surface of the electron transit layer 3 formed of GaN, the electronsupply layer 4 formed of AlGaN whose lattice constant is different isformed, and moreover, AlGaN is a ternary crystal, its crystallinity isnot necessarily satisfactory. Hence, when the electron supply layer 4 isthe uppermost surface of the nitride semiconductor layer, the surfacemorphology is not necessarily satisfactory, and the properties of thedevice are not stable accordingly. Hence, the cap layer 5 of the samecomposition as the electron transit layer 3 is deposited on the electronsupply layer 4, and thus the surface morphology of the nitridesemiconductor layer can be improved, with the result that the propertiesof the device can be enhanced. However, when the thickness of the caplayer 5 is excessively increased, since the effect of improving thesurface morphology is reduced, and the ohmic contacts of the sourceelectrode 6 and the drain electrode 7 are adversely affected, itsthickness is preferably 16 nm or less (more preferably 8 nm or less). Itmay be assumed that the cap layer 5 is part of the electron supplylayer, and therefore, it can be said that the cap layer 5 together withthe electron supply layer 4 form an electron supply layer.

In this preferred embodiment, the buffer layer 2 includes an AlN bufferlayer 21 in contact with the substrate 1 and an AlGaN buffer layer 22deposited on the AlN buffer layer 21. The thickness of the AlN bufferlayer 21 is, for example, about 0.1 μm. The AlGaN buffer layer 22includes a first AlGaN layer 221 deposited on the AlN buffer layer 21and a second AlGaN layer 222 deposited on the first AlGaN layer 221. Onthe second AlGaN layer 222, the electron transit layer 3 is formed. Thefirst AlGaN layer 221 and the second AlGaN layer 222 have differentcompositions. More specifically, the first AlGaN layer 221 and thesecond AlGaN layer 222 have different aluminum compositions. In otherwords, the aluminum composition x1 (0<x1<1) of Al_(x1)Ga_(1-x1)N formingthe first AlGaN layer 221 is larger than the aluminum composition x2(0<x2<x1<1) of Al_(x2)Ga_(1-x2)N forming the second AlGaN layer 222. Inother words, the AlGaN buffer layer 22 includes the first AlGaN layer221 and the second AlGaN layer 222 which are laminated in such an orderthat as they approach the electron transit layer 3, the aluminumcompositions are decreased. The total thickness of the first AlGaN layer221 and the second AlGaN layer 222 may be, for example, about 1 μm. Morespecifically, the thickness of the first AlGaN layer 221 may be about0.2 μm, and the thickness of the second AlGaN layer 222 may be about 0.8μm.

The first AlGaN layer 221 is a first aluminum composition AlGaN layer(high aluminum composition region) of a first aluminum composition, andthe second AlGaN layer 222 is a second aluminum composition AlGaN layer(low aluminum composition region) of a second aluminum compositionsmaller than the first aluminum composition. The AlGaN buffer layer 22can be said to be an AlGaN layer whose aluminum composition is adjustedsuch that as the AlGaN buffer layer 22 approaches the electron transitlayer 3 in the direction of the thickness of the layer from thesubstrate 1 toward the electron transit layer 3, the aluminumcomposition is decreased.

The nitride semiconductor layer formed on the substrate 1 is dividedinto a plurality of element regions DR. Each of the element regions DRis electrically separated from the other element regions DR with anelement separation layer 13. In this preferred embodiment, the elementseparation layer 13 passes through the cap layer 5, the electron supplylayer 4 and the electron transit layer 3 to reach the buffer layer 2.More specifically, the element separation layer 13 extends in thedirection of the thickness of the electron transit layer 3, and isformed to reach a position of a predetermined depth (for example, about1.1 μm) from the surface of the cap layer 5. In this preferredembodiment, the element separation layer 13 reaches a depth exceedingthe interface between the first AlGaN layer 221 and the second AlGaNlayer 222, and its bottom portion is located within the first AlGaNlayer 221. In this preferred embodiment, the element separation layer 13is a high resistance layer whose resistance is increased by breaking thecrystal structure of the nitride semiconductor through ion implantationand thereby causing a crystal defect.

The element separation layer 13 is formed so as to surround theindividual element regions DR. Within the element regions DR, the HEMT(high-electron-mobility transistor) structure is formed. In other words,within each of the element regions DR, the gate electrode 8 is locatedbetween the source electrode 6 and the drain electrode 7, and the gateelectrode 8 is opposite to the electron transit layer 3 (morespecifically, the two-dimensional electron gas 11) across the gateinsulating film 10.

On the element separation layer 13, an opening 13 a is formed. Thebottom surface and the side surface of the opening 13 a are also coveredwith the gate insulating film 10. On the element separation layer 13,via the gate insulating film 10, a gate wiring film 18 is arranged. Thegate wiring film 18 is connected to the gate electrode 8. In thispreferred embodiment, the gate electrodes 8 arranged in a plurality ofthe element regions DR are connected in common to the gate wiring film18.

FIG. 9 is a schematic plan view for illustrating the overallconfiguration of a chip of the nitride semiconductor device according tothis preferred embodiment, and shows the configuration by seeing throughthe interlayer insulating film 12. FIG. 10 is a schematic plan view forillustrating the arrangement of the element separation layer 13 and theelement regions DR. Furthermore, FIG. 11 is a schematic plan view forillustrating the arrangement of the source wiring film 16 and the drainwiring film 17. FIG. 12 is an enlarged plan view of a part of FIG. 9,and shows the configuration by seeing through the interlayer insulatingfilm 12 as in FIG. 9.

As shown most clearly in FIG. 10, the element region DR is a longrectangular (belt-shaped) region that extends in a predetermineddirection. The belt-shaped element regions DR extend parallel to eachother, and are formed in the shape of a stripe. The element separationlayer 13 (in FIG. 10, indicated by oblique lines for clarity) isarranged in the region other than the element regions DR so as to edgethe element regions DR. In other words, the element separation layer 13is arranged so as to surround each of the element regions DR and tosurround the element regions DR as a whole. The element region DR is anactive region that contributes to the operation of the device bymovement of electrons. By contrast, the region of the element separationlayer 13 is a high-resistance region that inhibits the movement ofelectrons, and a non-active region that does not contribute to theoperation of the device.

As shown most clearly in FIG. 11, on the interlayer insulating film 12,the source wiring film 16 and the drain wiring film 17 are formed so asto mesh with each other in a comb tooth shape. More specifically, thesource wiring film 16 includes a source pad portion 16A for externalconnection, a plurality of source bus connection portions 16B thatextend from the source pad portion 16A parallel to each other and aplurality of source branch-shaped portions 16C that protrude from thesource bus connection portions 16B. In a plan view, the source padportion 16A is formed in the shape of a long rectangle that is formedalong one side of the rectangular nitride semiconductor device. Thesource bus connection portions 16B are formed in the shape of longrectangles extending parallel to the element regions DR, and are formedon the element separation layer 13 between the adjacent element regionsDR. From a pair of sides of the source bus connection portion 16B alongits longitudinal direction, in a vertical direction with respect to thesides, the source branch-shaped portions 16C extend to both sides of theelement regions DR over the substantially entire width of the elementregions DR. Likewise, the drain wiring film 17 includes a drain padportion 17A for external connection, a plurality of drain bus connectionportions 17B that extend from the drain pad portion 17A parallel to eachother and a plurality of drain branch-shaped portions 17C that protrudefrom the drain bus connection portions 17B. The drain pad portion 17A isformed in the shape of a long rectangle that is formed along one side onthe opposite side to the source pad portion 16A of the nitridesemiconductor device. The drain bus connection portions 17B are formedin the shape of long rectangles extending parallel to the elementregions DR, and are formed on the element separation layer 13 betweenthe adjacent element regions DR. From a pair of sides of the drain busconnection portions 17B along its longitudinal direction, in a verticaldirection with respect to the sides, the drain branch-shaped portions17C extend to both sides of the element regions DR over thesubstantially entire width of the element regions DR.

In the region on the element separation layer 13 located between theelement regions DR, the source bus connection portions 16B and the drainbus connection portions 17B are alternately arranged, and thus thesource bus connection portions 16B and the drain bus connection portions17B mesh with each other in a comb tooth shape. Furthermore, within eachof the element regions DR, the source branch-shaped portions 16C and thedrain branch-shaped portions 17C are alternately arranged in thelongitudinal direction of the element region DR, and they mesh in a combtooth shape.

On the other hand, the gate wiring film 18 includes a gate pad portion18A and a gate bus connection portion 18B. In the vicinity of one endportion of the source pad portion 16A, a substantially rectangularcutout is formed, and the gate pad portion 18A is arranged so as tocorrespond to the cutout. In the interlayer insulating film 12, anopening 18 b (see FIG. 11) for exposing the gate pad portion 18A isformed. The gate bus connection portion 18B is routed and arranged onthe element separation layer 13 so as to be located between the sourcewiring film 16 and the element region DR in a plan view. The gate busconnection portion 18B extends along one side of the element region DRalong its longitudinal direction. A plurality of gate electrodes 8extend in the shape of branches parallel to each other, from the gatebus connection portion 18B toward the element region DR, along adirection perpendicular to the longitudinal direction of the elementregion DR, and are spread over the entire width of the element regionDR.

The source wiring film 16 is formed on the interlayer insulating film 12whereas the gate wiring film 18 is formed below the interlayerinsulating film 12. Hence, although they may be arranged to overlap eachother, in this preferred embodiment, in a plan view, the source padportion 16A, the source bus connection portions 16B and the drain wiringfilm 17 are arranged so as not to overlap the gate wiring film 18.However, the source branch-shaped portions 16C are arranged to passthrough an upper portion of the gate wiring film 18 and reach theelement regions DR.

The source electrodes 6 respectively formed in a plurality of theelement regions DR are connected in common to the source wiring film 16.The drain electrodes 7 respectively formed in a plurality of the elementregions DR are connected in common to the drain wiring film 17.Furthermore, as described above, the gate electrodes 8 respectivelyformed in a plurality of the element regions DR are connected in commonto the gate wiring film 18. In this way, the HEMT elements formed in theelement regions DR are connected to each other with the source wiringfilm 16, the drain wiring film 17 and the gate wiring film 18. Hence,the source wiring film 16, the drain wiring film 17 and the gate wiringfilm 18 are an example of an element-to-element wiring that connects aplurality of elements. In this preferred embodiment, the HEMT elementshaving the common function are formed in a plurality of the elementregions DR, and they are connected to each other with theelement-to-element wiring.

The element separation layer 13 surrounds the individual element regionsDR, and surrounds, as a whole, a plurality of element regions DR whichinclude a plurality of elements, respectively, connected to each otherthrough the element-to-element wiring formed by the source wiring film16, the drain wiring film 17 and the gate wiring film 18.

FIG. 13 is a partially enlarged plan view for illustrating theconfiguration within the element region DR. FIG. 8 shows a cross sectiontaken along cut surface line VIII-VIII in FIG. 13.

Within the element region DR, the source branch-shaped portion 16C isconnected via the source contact hole 16 a (see FIG. 8) to the sourceelectrode 6, and the drain branch-shaped portion 17C is connected viathe drain contact hole 17 a (see FIG. 8) to the drain electrode 7.

The source electrode 6 and the drain electrode 7 are formed in the shapeof a belt extending in a width direction perpendicular to thelongitudinal direction of the element region DR. A plurality of sourceelectrodes 6 and a plurality of drain electrodes 7 are alternatelyaligned, with a predetermined distance, one by one in the longitudinaldirection of the element region DR. Furthermore, each of the gateelectrodes 8 is arranged between the source electrode 6 and the drainelectrode 7. The gate electrode 8 extends in the direction of the widthof the element region DR over the entire width of the element region DR.More specifically, the opening 8 a (see FIG. 8) in which the gateelectrode 8 is embedded is spread over the entire width of the elementregion DR. In this way, the gate electrode 8 surrounds the sourceelectrode 6 together with the element separation layer 13. In this way,when a predetermined interruption voltage is applied to the gateelectrode 8, the two-dimensional electron gas 11 can be separated intothe side of the source electrode 6 and the side of the drain electrode 7immediately below the gate electrode 8. The source electrode 6 and thedrain electrode 7 do not need to extend over the entire width of theelement region DR.

The distance between the gate electrode 8 and the source electrode 6 isshorter than the distance between the gate electrode 8 and the drainelectrode 7. In other words, in the vicinity of the source electrode 6,the gate electrode 8 is formed along the source electrode 6. In thisway, the electric field between the gate electrode 8 and the drainelectrode 7 is alleviated, and the voltage resistance is enhanced.

As shown in FIG. 8, a positive bias is applied to the drain electrode 7with the potential of the source electrode 6 being a reference (zeropotential). When in this state, a negative control voltage is applied tothe gate electrode 8, the two-dimensional electron gas 11 disappearsimmediately below the gate electrode 8, with the result that aninterruption occurs between the source and the drain. When the negativecontrol voltage is removed from the gate electrode 8, thetwo-dimensional electron gas 11 appears immediately below the gateelectrode 8, and this serves as a channel to make a connection betweenthe source and the drain.

FIG. 14A is a band diagram showing energy levels at a plurality of partsin the direction (normal direction to the major surface of the substrate1) of the thickness of the nitride semiconductor device. Morespecifically, FIG. 14A is a band diagram on the example where the AlGaNbuffer layer 22 is formed with the first AlGaN layer 221 (for example,Al_(0.25)Ga_(0.75)N) of high aluminum composition and the second AlGaNlayer 222 (for example, Al_(0.09)Ga_(0.92)N) of low aluminumcomposition. FIG. 14B is a similar band diagram on the structure of thecomparative example where the AlGaN buffer layer 22 is formed with onlythe AlGaN layer (for example, Al_(0.25)Ga_(0.75)N) of high aluminumcomposition. Furthermore, FIG. 14C is a similar band diagram when in thecomparative example described above, the thickness of the AlGaN bufferlayer 22 formed with a single layer of the AlGaN layer (for example,Al_(0.25)Ga_(0.75)N) is decreased and the thickness of the electrontransit layer 3 is increased (for example, about 1 μm).

As shown in FIGS. 14A, 14B and 14C, the energy level within the AlNbuffer layer 21 is rapidly increased as approaching from the substrate 1(for example, silicon substrate) toward the AlGaN buffer layer 22. Onthe other hand, in the comparative example shown in FIG. 14B, the energylevel in the AlGaN buffer layer 22 formed with the AlGaN single layer ofhigh aluminum composition draws a large downwardly convex parabola toconnect between the AlN buffer layer 21 and the electron transit layer3. Hence, in the vicinity of the interface between the AlN buffer layer21 and the AlGaN buffer layer 22, a sharp triangular potential barrieris formed. Electrons pass through the triangular potential barrier bythe quantum tunneling effect, and thus a leak current is produced.

In the comparative example shown in FIG. 14C, since the electron transitlayer 3 formed of GaN is increased in thickness, although the lowestenergy level of the AlGaN buffer layer 22 is high, as in the comparativeexample shown in FIG. 14B, in the vicinity of the interface between theAlN buffer layer 21 and the AlGaN buffer layer 22, a sharp triangularpotential barrier appears. Hence, as in the case of the comparativeexample shown in FIG. 14B, a leak current is produced by the quantumtunneling effect.

In the inventive example shown in FIG. 14A, in addition to the firstAlGaN layer 221 of high aluminum composition, the second AlGaN layer 222of low aluminum composition is provided, and thus the energy level inthe part of the first AlGaN layer 221 (high aluminum composition region)is increased. Consequently, between the AlN buffer layer 21 and thesecond AlGaN layer 222 (low aluminum composition region), a trapezoidalpotential barrier is formed. Since the trapezoidal potential barrierprevents the passage of electrons caused by the quantum tunnelingeffect, a depletion layer in the vicinity of the interface between theAlN buffer layer 21 and the AlGaN buffer layer 22 is widened, and thusit is possible to increase the resistance. In this way, it is possibleto reduce or prevent the leak current.

FIGS. 15A, 15B and 15C are diagrams for illustrating features on thethickness of the electron transit layer 3 formed with the GaN layer, andare optical micrographs of the surface of the GaN layer epitaxiallygrown to various thicknesses. FIG. 15A shows a state of the surface whenthe GaN layer is epitaxially grown until its thickness reaches 0.1 μm,FIG. 15B shows a state of the surface when the GaN layer is epitaxiallygrown until its thickness reaches 0.4 μm, and FIG. 15C shows a state ofthe surface when the GaN layer is epitaxially grown until its thicknessreaches 1.0 μm.

In the GaN layer (FIG. 15A) having a thickness of 0.1 μm, hexagonalprojections and recesses appear in the surface, and thus sufficientsmoothness cannot be obtained. In the GaN layers (FIGS. 15B and 15C)having a thickness of 0.4 μm and a thickness of 1.0 μm, the surface issmooth, smoothness necessary for obtaining high electron mobility isobtained and the GaN layer (FIG. 15C) having a thickness of 1.0 μmprovides a more smooth surface.

The two-dimensional electron gas 11 is formed near the surface of theelectron transit layer 3, and electrons forming the two-dimensionalelectron gas 11 contribute to the operation of the device. Thus, thesmoothness of the surface of the electron transit layer 3 affects themobility of electrons. Hence, the electron transit layer 3 is formedwith a GaN layer whose thickness is 0.4 μm (400 nm) or more, and thus itis possible to make the surface of the electron transit layer 3sufficiently smooth, and it is possible to provide a device capable ofperforming a high-speed operation accordingly.

FIGS. 16A to 16I are cross-sectional views showing a method ofmanufacturing the nitride semiconductor device in order of steps.

First, as shown in FIG. 16A, on the substrate 1, the buffer layer 2, theelectron transit layer 3, the electron supply layer 4 and the cap layer5 are sequentially epitaxially grown. These may be, for example, nitridesemiconductor crystal layers having a c-plane as the main surface. Inthe epitaxial growth of the buffer layer 2, first, the AlN buffer layer21 is grown, then the first AlGaN layer 221 formed of AlGaN of highaluminum composition is grown and then the second AlGaN layer 222 formedof AlGaN of aluminum composition lower than it is grown. In other words,the AlGaN crystal is epitaxially grown on the substrate 1 such that thealuminum composition is relatively high in a region close to thesubstrate 1 and is relatively low in a region away from the substrate 1,with the result that the AlGaN buffer layer 22 is formed. The flow rateof raw gas at the time of epitaxial growth is controlled, and thus it ispossible to control the composition of each layer. After the step ofepitaxially growing the nitride semiconductor described above, withinthe same crystal growth equipment, the lower layer 91 of the passivationfilm 9 formed of SiN is formed over the entire surface. The passivationfilm 91 may be formed by another film formation method, for example, aplasm CVD method (plasma chemical vapor deposition method).

Next, as shown in FIG. 16B, on the lower layer 91, a resist mask 31 isformed that has an opening in a position corresponding to the contactholes 6 a and 7 a for the source electrode 6 and the drain electrode 7.The contact holes 6 a and 7 a are formed in the lower layer 91 byetching via the resist mask 31.

Then, as shown in FIG. 16C, after the removal of the resist mask 31, thesource electrode 6 and the drain electrode 7 which are respectivelyembedded in the contact holes 6 a and 7 a are formed. The sourceelectrode 6 and the drain electrode 7 are formed of a material that canbe in ohmic junction to the two-dimensional electron gas 11 via the caplayer 5 and the electron supply layer 4. More specifically, a multilayerelectrode film in which a Ti layer in contact with the cap layer 5 andan Al layer formed on the Ti layer are laminated may be used. Forexample, a resist having an opening in a position corresponding to thesource electrode 6 and the drain electrode 7 is formed, an electrodefilm is formed (for example, formed by a sputtering method) therefromand an unnecessary part is lifted off together with the resist, with theresult that the source electrode 6 and the drain electrode 7 of apredetermined pattern may be formed. Furthermore, thermal processing isperformed, and thus the source electrode 6 and the drain electrode 7 canbe brought into ohmic contact with the two-dimensional electron gas 11.

Next, as shown in FIG. 16D, the upper layer 92 (for example, a SiNlayer) of the passivation film 9 is formed over the entire surface bythe plasma CVD method. Next, as shown in FIG. 16E, a resist mask 32 of apattern corresponding to the element regions DR is formed. In otherwords, the resist mask 32 is formed into a pattern having an opening ina region where the element separation layer 13 is to be formed. Theresist mask 32 is used as an etching mask, and thus the passivation film9 is etched. In this way, in the passivation film 9, the opening 13 a isformed to expose the region where the element separation layer 13 is tobe formed.

Furthermore, as shown in FIG. 16F, the resist mask 32 is used as a mask,and ion implantation for forming the element separation layer 13 isperformed. In the ion implantation, an ion species is used in which itsmaterial is an element whose mass number is less than 10 but more than2. As such an ion species, a helium ion can be illustrated. The heliumion can be made to deeply reach the nitride semiconductor crystal, andcan cause sufficient crystal defects in the crystal structure. Thepassivation film 9 covers the element regions DR, prevents the ion fromreaching the element regions DR and thereby protects the element regionsDR.

The ion implantation is preferably performed by oblique implantationfrom a direction which is inclined with respect to the normal direction(that is, the direction of the thickness of the electron transit layer3) to the major surface of the substrate 1. When the major surface ofthe electron transit layer 3 etc. is the c-plane, the ion implantationis performed from a direction which is inclined with respect to a c-axisdirection. The inclination angle of the ion implantation direction withrespect to the normal direction (the direction of the thickness of theelectron transit layer 3, for example, the c-axis direction) to themajor surface of the substrate 1 is preferably 5 to 10 degrees. By theion implantation from the inclination direction described above, it ispossible to efficiently make the ions collide with the constituentelements of the nitride semiconductor having a crystal structure ofhexagonal crystal, and thus defects are formed in the crystal structure,with the result that it is possible to increase the resistance.

The ion implantation is performed by multistage implantation using aplurality of acceleration energy settings. In this way, the crystalstructure of the nitride semiconductor layer can be broken over the widerange of depth from the cap layer 5 to the buffer layer 2, and thus itis possible to form the element separation layer 13 formed with a highresistance layer over the range of depth.

After the ion implantation, the resist mask 32 is separated. Next, asshown in FIG. 16G, a resist mask 33 is formed such that the resist mask33 has an opening corresponding to the position where the gate electrode8 is formed. By etching via the resist mask 33, the opening 8 a thatreaches the cap layer 5 is formed in the passivation film 9.

Thereafter, as shown in FIG. 16H, the resist mask 33 is removed, andthen the gate insulating film 10 made of, for example, alumina is formedby, for example, the ALD method. Hence, the gate insulating film 10 iscontinuous so as to be in contact with the cap layer 5 on the bottomsurface of the opening 8 a and the upper surface of the elementseparation layer 13, to further cover the side surface of the openings 8a and 13 a, and to cover the surface of the passivation film 9 outsidethe openings 8 a and 13 a. In this state, furthermore, the gateelectrode 8 and the gate wiring film 18 are formed with the sameelectrode film. The electrode film may be a multilayer electrode filmhaving a lower layer that is made of Ni or Pt and an upper layer that islaminated on the lower layer and that is made of Au or Al. Preferably,when the gate electrode 8 and the gate wiring film 18 are formed, forexample, the formation of the resist pattern and the formation (forexample, formation by the sputtering method) of the electrode filmcovering the upper part thereof are sequentially performed, andthereafter an unnecessary part of the electrode film is lifted offtogether with the resist pattern.

Next, as shown in FIG. 16I, for example, the interlayer insulating film12 made of SiO₂ is formed by the plasma CVD method. Then, in theinterlayer insulating film 12, in positions immediately above the sourceelectrode 6 and the drain electrode 7, the source contact hole 16 a andthe drain contact hole 17 a passing through the interlayer insulatingfilm 12 and the upper layer 92 of the passivation film 9 are formed byetching.

Thereafter, the wiring film embedded in the source contact hole 16 a andthe drain contact hole 17 a are formed, and the wiring film is patternedon the source wiring film 16 and the drain wiring film 17 by etching,and thus it is possible to obtain the nitride semiconductor device ofthe structure shown in FIG. 8.

Furthermore, thermal processing is performed, and thus it is possible toreduce the contact resistance between the source electrode 6 and thesource wiring film 16 and the contact resistance between the drainelectrode 7 and the drain wiring film 17.

FIG. 17 is a diagram for illustrating the details of the multistage ionimplantation for forming the element separation layer 13. Specifically,FIG. 17 shows the dependence of the depth of crystal defects caused bythe impact of ions implanted when the ions are implanted from adirection inclined by 5 to 10 degrees with respect to the normaldirection (for example, the c-axis direction) to the major surface (forexample, the c-plane) for the nitride semiconductor crystal.

Helium ions are implanted into the nitride semiconductor crystal in fivestages from the first to fifth stages under the conditions of table 1below, and thus by the ion implantation in each stage, helium ions reachthe depth different depending on acceleration energy. In this way, thedistribution of the crystal defects, in the direction of depth, formedin the nitride semiconductor crystal shows a profile having peaks atdifferent depths from the surface. A combination profile obtained bycombining the profiles of the distributions of crystal defects caused bythe ion implantation in the first to fifth stages has a crystal defectdensity of a predetermined value or more until a depth of about 1.1 μm.Hence, it is found that it is possible to increase the resistance untilsuch a depth.

TABLE 1 Acceleration energy Dose amount (keV) (1/cm²) First stage 3502.6 × 10¹⁴ Second stage 240 1.3 × 10¹⁴ Third stage 150 1.3 × 10¹⁴ Fourthstage 70 1.3 × 10¹⁴ Fifth stage 20 9.6 × 10¹³

Since the ions are easily made to collide with the atoms of the nitridesemiconductor crystal by the ion implantation from the inclineddirection, it is possible to accurately control the depth of the ionimplantation. In this way, since the element separation layer 13 of highresistance is reliably formed, it is possible to provide the nitridesemiconductor device in which the leak current is reduced. Inparticular, by setting the inclination angle at 5 to 10 degrees, thedepth of the ion implantation is accurately controlled, with the resultthat it is possible to realize the element separation layer 13 of highresistance.

FIG. 18 shows leak characteristics in an off-state. More specifically,FIG. 18 shows results obtained by measuring the drain current Id(source-to-drain current) and the gate current Ig (source-to-gatecurrent) in a state where an off voltage is applied to the gateelectrode 8 to cause an interruption between the source and the drain.The horizontal axis represents a bias between the source electrode 6 andthe drain electrode 7.

Curves C1 d and C1 g represent the results of experiments on a structure(Comparative Example 1) in which the element separation layer 13 isomitted from the configuration of FIG. 8 and in which the second AlGaNlayer 222 is omitted. Curves C2 d and C2 g represent the results ofexperiments on a structure (Comparative Example 2) in which the secondAlGaN layer 222 is omitted from the configuration of FIG. 8. Curves C3 dand C3 g represent the results of experiments on a structure(Comparative Example 3) in which the element separation layer 13 isomitted from the configuration of FIG. 8. Furthermore, curves Ed and Egrepresent the results of experiments on a structure (Inventive Example)according to the preferred embodiment shown in FIG. 8.

It is found from the comparison of Comparative Examples 1 and 2 that theprovision of the element separation layer 13 makes it possible to reducethe drain leak current by about two significant digits and to reduce thegate leak current by about four significant digits. Furthermore, it isfound from the comparison of Comparative Example 2 and Inventive Exampleand the comparison of Comparative Examples 1 and 3 that the provision ofthe second AlGaN layer 222 formed with the AlGaN layer of low aluminumcomposition between the first AlGaN layer 221 formed with the AlGaNlayer of high aluminum composition and the electron transit layer 3makes it possible to reduce the drain leak current and the gate leakcurrent. It is found from the comparison of Comparative Example 3 andInventive Example that the provision of the second AlGaN layer 222formed with the AlGaN layer of low aluminum composition between thefirst AlGaN layer 221 formed with the AlGaN layer of high aluminumcomposition and the electron transit layer 3 and the structure where theelement separation layer 13 is provided make it possible to minimize thedrain leak current and the gate leak current.

As described above, in the configuration of this preferred embodiment,the AlGaN buffer layer 22 is formed between the substrate 1 and theelectron transit layer 3, and furthermore, the element separation layer13 passing through the electron supply layer 4 and the electron transitlayer 3 reaches the AlGaN buffer layer 22. The element separation layer13 is formed with a region whose resistance is increased by causingcrystal defects in the nitride semiconductor through the ionimplantation. With the element separation layer 13, it is possible toseparate the elements in the direction along the direction of the mainsurface of the substrate 1.

The AlGaN buffer layer 22 includes the first AlGaN layer 221 of highaluminum composition and the second AlGaN layer 222 of low aluminumcomposition, and the second AlGaN layer 222 is arranged in a regionclose to the electron transit layer 3 as compared with the first AlGaNlayer 221. Between the second AlGaN layer 222 of low aluminumcomposition and the substrate 1, the first AlGaN layer 221 of highaluminum composition intervenes, and thus variations in energy level inthe direction of the thickness of the AlGaN buffer layer 22 are reduced.Hence, the thickness of the potential barrier is increased, and thus itis possible to reduce the leak caused by the quantum tunneling effect.In this way, it is possible to interrupt the leak path via the bufferlayer 2.

Consequently, with the element separation layer formed to reach thebuffer layer 2 by the ion implantation and the AlGaN buffer layer 22obtained by combining the high aluminum composition region and the lowaluminum composition region, it is possible to provide the nitridesemiconductor device in which the leak current is reduced.

Since the element separation layer 13 is formed so as to surround theindividual element regions DR, it is possible to reduce or prevent theleak inside and outside the element regions DR. Furthermore, since theelement separation layer 13 totally surrounds a plurality of elementregions DR connected in common with the wiring films 16, 17 and 18forming the element-to-element wiring, while the leak in each of theelement regions is being reduced, a plurality of elements formed in aplurality of element regions are coupled, with the result that it ispossible to form a large element which realizes the desired function andit is possible to reduce the leak current in the large element as awhole.

In this preferred embodiment, since the space on the element separationlayer 13 is utilized to route the wiring films 16, 17 and 18, theconfiguration is advantageous in high integration. Moreover, since theelement separation layer 13 below the wiring films 16, 17 and 18 is ahigh resistance layer thick enough to reach the buffer layer 2, it ispossible to reduce the parasitic capacitance, with the result that it ispossible to perform a high-speed operation accordingly. Hence, while theleak in each of the element regions is being reduced, it is possible toprovide the nitride semiconductor device capable of performing ahigh-speed operation.

The element separation layer 13 is a high resistance layer in which anelement whose mass number is less than 10 but more than 2 is used as itsmaterial and which is formed by the ion multistage implantation throughoblique implantation. Hence, even when the thickness of the electrontransit layer 3 formed with the GaN layer is increased to enhance theelectron mobility, and the thickness of the AlGaN buffer layer 22 isincreased to reduce the leak current, it is possible to form the elementseparation layer 13 having a high resistance in a position which is deepto reach the buffer layer 2. Since the ion species using, as thematerial, the element whose mass number is sufficient to producesufficient crystal defects for the crystal structure of the nitridesemiconductor is used, for example, when the thermal processing thatbrings the source electrode 6 and the drain electrode 7 into ohmiccontact with the nitride semiconductor layer is performed, the crystaldefects are prevented from being cured. Hence, since the elementseparation layer 13 has a sufficiently high resistance value in anydepth position, it is possible to effectively reduce the leak current.

Variations of this preferred embodiment are possible as follows.

For example, although in the preferred embodiment described above, onthe first AlGaN layer 221 of high aluminum composition, the second AlGaNlayer 222 of low aluminum composition is deposited, a third AlGaN layerof aluminum composition further lower than that of the second AlGaNlayer 222 may be made to intervene between the second AlGaN layer 222and the electron transit layer 3. In other words, between the AlN bufferlayer 21 and the electron transit layer 3, two or more AlGaN layers maybe made to intervene, and its aluminum composition may be monotonicallylowered from the AlN buffer layer 21 toward the electron transit layer3. Furthermore, between the AlN buffer layer 21 and the electron transitlayer 3, an Al_(X)Ga_(1-X)N layer (0<x≤1) in which the aluminumcomposition x is continuously and monotonically reduced toward theelectron transit layer 3 may be made to intervene.

The AlN buffer layer 21 may be omitted. However, in such a case, the Gacomposition of the AlGaN layer in the interface with the substrate 1 ispreferably zero.

Although in the preferred embodiment described above, the cap layer 5 isprovided on the surface of the electron supply layer 4, the cap layer 5may be omitted.

In the preferred embodiment described above, the elements having thecommon function are formed in a plurality of element regions DR, andthey are connected with the element-to-element wiring (the wiring films16, 17 and 18), with the result that the large element is formed.However, elements having different functions are formed in a pluralityof element regions, and they are connected with the element-to-elementwiring, with the result that the large element may be formed.Furthermore, a plurality of elements that are formed in the elementregions according to the necessary function may include a first element,a second element having the common function to the first element and athird element having a different function from the first element. Inthis way, by combining the element of the common function and theelement of the different function, it is possible to form a largeelement having a desired function.

Furthermore, the features of the first preferred embodiment (FIGS. 1 to7) may be incorporated into the second preferred embodiment.

Although the preferred embodiments of the present invention have beendescribed in detail, these are simply specific examples used forclarifying the technical details of the present invention, the presentinvention should not be interpreted to be limited to these specificexamples and the scope of the present invention is limited only by thescope of claims attached.

This application corresponds to Japanese Patent Application No.2012-226256 and Japanese Patent Application No. 2012-272725 respectivelyfiled in Japan Patent Office on Oct. 11, 2012 and Dec. 13, 2012, alldisclosures of which are incorporated herein by reference.

DESCRIPTION OF THE SYMBOLS

-   102 Buffer layer-   103 Electron transit layer-   104 Electron supply layer-   105 Passivation film-   106 Source electrode-   107 Drain electrode-   108 Gate electrode-   109 Recess-   109 a Bottom portion-   109 b Side wall portion-   110 Gate insulating film-   111 Thermal oxide film-   111 a Bottom portion covering portion-   111 b Side wall covering portion-   115 Two-dimensional electron gas-   DR Element region-   1 Substrate-   2 Buffer layer-   3 Electron transit layer-   4 Electron supply layer-   5 Cap layer-   6 Source electrode-   7 Drain electrode-   8 Gate electrode-   9 Passivation film-   10 Gate insulating film-   11 Two-dimensional electron gas-   12 Interlayer insulating film-   13 Element separation layer-   16 Source wiring film-   17 Drain wiring film-   18 Gate wiring film-   21 AlN buffer layer-   22 AlGaN buffer layer-   221 First AlGaN layer (high aluminum composition)-   222 Second AlGaN layer (low aluminum composition)

What is claimed is:
 1. A nitride semiconductor device comprising: asubstrate; an electron transit layer that is formed on the substrate,and that is formed of a nitride semiconductor; an electron supply layerthat is formed on the electron transit layer, and that is formed of anitride semiconductor whose composition is different from the electrontransit layer; an AlGaN buffer layer that intervenes between thesubstrate and the electron transit layer, and that includes a highaluminum composition region whose aluminum composition is relativelyhigh and a low aluminum composition region whose aluminum composition islower than the high aluminum composition region and which is arranged ina region close to the electron transit layer as compared with the highaluminum composition region; and an element separation layer that isformed with a region whose resistance is increased by causing a crystaldefect through ion implantation, and that passes through the electronsupply layer, the electron transit layer, and the low aluminumcomposition region to reach the high aluminum composition region, abottom portion of the element separation layer being located within thehigh aluminum composition region.
 2. The nitride semiconductor deviceaccording to claim 1, wherein the AlGaN buffer layer is an AlGaN layerwhose aluminum composition is adjusted such that as the AlGaN bufferlayer approaches the electron transit layer in a direction of a layerthickness from the substrate toward the electron transit layer, thealuminum composition is decreased.
 3. The nitride semiconductor deviceaccording to claim 1, wherein the AlGaN buffer layer includes a firstaluminum composition AlGaN layer that has a first aluminum compositionand a second aluminum composition AlGaN layer that is deposited on aside of the electron transit layer with respect to the first aluminumcomposition AlGaN layer and that has a second aluminum composition lowerthan the first aluminum composition, and the high aluminum compositionregion includes the first aluminum composition AlGaN layer, and the lowaluminum composition region includes the second aluminum compositionAlGaN layer.
 4. The nitride semiconductor device according to claim 1,further comprising: an AlN buffer layer that intervenes between theAlGaN buffer layer and the substrate.
 5. The nitride semiconductordevice according to claim 1, wherein the element separation layer isformed so as to surround an element region.
 6. The nitride semiconductordevice according to claim 1, further comprising: a wiring that isarranged on the element separation layer.
 7. The nitride semiconductordevice according to claim 1, wherein the element separation layer isformed so as to separate a plurality of element regions, and the nitridesemiconductor device further comprises an element-to-element wiring thatconnects between a plurality of elements respectively formed in theelement regions.
 8. The nitride semiconductor device according to claim7, wherein the elements respectively formed in the element regionsinclude two or more elements having different functions.
 9. The nitridesemiconductor device according to claim 7, wherein the elementsrespectively formed in the element regions include two or more elementshaving a common function.
 10. The nitride semiconductor device accordingto claim 7, wherein the element separation layer is formed so as tosurround the element regions of the elements connected with theelement-to-element wiring.
 11. The nitride semiconductor deviceaccording to claim 1, wherein the element separation layer is a highresistance layer that is formed by ion implantation using, as amaterial, an element whose mass number is less than 10 but more than 2.12. The nitride semiconductor device according to claim 1, wherein theelement separation layer is a high resistance layer that is formed byimplantation of a helium ion.
 13. The nitride semiconductor deviceaccording to claim 1, wherein the element separation layer is a highresistance layer that is formed by ion implantation using a plurality ofacceleration energies.
 14. The nitride semiconductor device according toclaim 1, wherein the element separation layer is a high resistance layerthat is formed by ion implantation from a direction inclined withrespect to a direction of thickness of the electron transit layer. 15.The nitride semiconductor device according to claim 14, wherein aninclination angle of the ion implantation direction with respect to thedirection of the thickness of the electron transit layer is 5 to 10degrees.
 16. The nitride semiconductor device according to claim 1,wherein the electron transit layer is formed of GaN, and the electronsupply layer is formed of AlGaN.
 17. The nitride semiconductor deviceaccording to claim 1, wherein the electron transit layer is a GaN layerhaving a thickness of 400 nm or more.
 18. The nitride semiconductordevice according to claim 1, further comprising: a source electrode anda drain electrode that are arranged at an interval on the electronsupply layer; and a gate electrode that is arranged opposite to theelectron transit layer between the source electrode and the drainelectrode.
 19. The nitride semiconductor device according to claim 18,wherein the gate electrode is formed so as to surround the sourceelectrode together with the element separation layer.
 20. The nitridesemiconductor device according to claim 18, further comprising: aninterlayer insulating film that covers the source electrode, the drainelectrode and the gate electrode; a source wiring film that is connectedto the source electrode via a source contact hole passing through theinterlayer insulating film and that is arranged on the interlayerinsulating film; and a drain wiring film that is connected to the drainelectrode via a drain contact hole passing through the interlayerinsulating film and that is arranged on the interlayer insulating film,wherein the source wiring film and the drain wiring film are formed insuch a pattern that on the interlayer insulating film, the source wiringfilm and the drain wiring film mesh with each other in a comb toothshape.
 21. The nitride semiconductor device according to claim 18, whichis the nitride semiconductor device further comprising a wiring that isarranged on the element separation layer, wherein the wiring includes agate wiring connected to the gate electrode.
 22. A method ofmanufacturing a nitride semiconductor device, the method comprising: astep of forming an AlGaN buffer layer by epitaxially growing an AlGaNcrystal on a substrate such that an aluminum composition is relativelyhigh in a high aluminum composition region close to the substrate andthe aluminum composition is relatively low in a low aluminum compositionregion away from the substrate; a step of forming an electron transitlayer by epitaxially growing a nitride semiconductor on the AlGaN bufferlayer; a step of forming an electron supply layer on the electrontransit layer by epitaxially growing a nitride semiconductor layer whosecomposition is different from the electron transit layer; and a step offorming a high resistance element separation layer that passes throughthe electron supply layer, the electron transit layer, and the lowaluminum composition region to reach the high aluminum compositionregion by breaking a crystal structure through ion implantation on theelectron supply layer, the electron transit layer, and the high aluminumcomposition region, wherein a bottom portion of the high resistanceelement separation layer is located within the high aluminum compositionregion.
 23. The method of manufacturing a nitride semiconductor deviceaccording to claim 22, wherein the step of forming the AlGaN bufferlayer includes a step of forming a first aluminum composition AlGaNlayer of a first aluminum composition and a step of forming, in a higherposition than a position of the first aluminum composition AlGaN layer,a second aluminum composition AlGaN layer of a second aluminumcomposition lower than the first aluminum composition.
 24. The method ofmanufacturing a nitride semiconductor device according to claim 22, themethod further comprising: a step of forming an AlN buffer layer on thesubstrate before the step of forming the AlGaN buffer layer such thatthe AlGaN buffer layer is formed on the AlN buffer layer.
 25. The methodof manufacturing a nitride semiconductor device according to claim 22,wherein the element separation layer is formed so as to surround anelement region.
 26. The method of manufacturing a nitride semiconductordevice according to claim 22, wherein the element separation layer isformed so as to separate a plurality of element regions, and the methodof manufacturing a nitride semiconductor device further comprises a stepof forming an element-to-element wiring that connects between elementsrespectively formed in the element regions.
 27. The method ofmanufacturing a nitride semiconductor device according to claim 22,wherein the ion implantation is performed using, as a material, anelement whose mass number is less than 10 but more than
 2. 28. Themethod of manufacturing a nitride semiconductor device according toclaim 22, wherein the ion implantation is performed using helium ions.29. The method of manufacturing a nitride semiconductor device accordingto claim 22, wherein the ion implantation is performed using a pluralityof acceleration energies.
 30. The method of manufacturing a nitridesemiconductor device according to claim 22, wherein the ion implantationis performed from a direction inclined with respect to a major surfaceof the substrate.
 31. The method of manufacturing a nitridesemiconductor device according to claim 30, wherein the ion implantationis performed from a direction inclined at an angle of 5 to 10 degreeswith respect to a normal direction to the major surface of thesubstrate.
 32. The method of manufacturing a nitride semiconductordevice according to claim 22, wherein the step of forming the electrontransit layer includes a step of epitaxially growing a GaN layer havinga thickness of 400 nm or more.